LIBRARY OF CONGRESS. 



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UNITED STATES OF AMERICA. 



SKIASCOPY 



AND ITS 



Practical Application to the 
Study of Refraction 



BY 



' Edward Jackson, a. m., m. d., 

PROFESSOR OP DISEASES OF THE EYE IN THE PHILADELPHIA POLYCLINIC AND COLLEGE FOR 

GRADUATES IN MEDICINE ; SURGEON TO WILLS EYE HOSPITAL; CHAIRMAN OF THE 

SECTION ON OPHTHALMOLOGY OF THE AMERICAN MEDICAL ASSOCIATION; 

MEMBER OF THE AMERICAN OPHTHALMOLOGICAL SOCIETY J 

ETC., ETC. 



WITH TWENTY-SIX ILLUSTRATIONS 



PHILADELPHIA: 
THE EDWARDS & DOCKEFf CO. 

1895 




Yf/o ^-'- 






COPYRIGHT, 1895 
BY EDWARD JACKSON 



CONTENTS 



PREFACE 



CHAPTER I.— History, Name, Difficulties and Study 

History 

Name 

Difficulties 

How to Study the Test 



CHAPTER II.— General Optical Principles. 
The reversal of movement 
Real Movement of light on retina, Plane Mirror 
Real Movement of light on retina, Concave Mirror 
Apparent Movement of light in pupil 
Rapidity of Movement of light on the retina 
Magnification of the retina 
Form of the light area .... 
Brilliancy of light in the pupil 
Finding the point of reversal . 



CHAPTER III— Conditions of Accuracy. 

The Source of light 

Focusing of light on the retina 
Position of greatest accuracy . 
Irregularities of the Media and Surfaces 
Distance of Surgeon from Patient . 

CHAPTER IV.— Regular Astigmatism. 

Two points of reversal 

The Band-like appearance 

Changes in the Light area at different distances 
Direction of movement of the band .... 

CHAPTER V. — Aberration and Irregular Astigmatism. 
Appearance of Irregular Astigmatism . 
Symmetrical Aberration ....... 

The Visual Zone 

Appearance of Positive Aberration ..... 



7 

10 
ii 
13 

19 
23 
24 
26 
28 
3o 
32 
33 
34 

36 
38 
40 
4i 
43 

46 
47 
53 

55 

56 
58 
59 
60 



Appearances of Negative Aberration ..... 63 

Appearance of Conical Cornea 65 

Scissors-like' Movement . . . , , . . . .68 

CHAPTER VI.— Practical Application with Plane Mirror. 

Position and Arrangement of Light . . , . • 71 

Hyperopia . . . . , .72 

Myopia . ... * 74 

Emmetropia 76 

Regular Astigmatism 77 

Aberration and Irregular Astigmatism . . 84 

Measurement of Accommodation 86 

CHAPTER VII. — Practical Application with Concave Mirror. 

The Source of Light 89 

Hyperopia .......... 90 

Myopia 91 

Emmetropia ... ...... 93 

Regular Astigmatism ........ 93 

Aberration and Irregular Astigmatism ..... 99 

Measurement of Accommodation 99 

CHAPTER VIII —General Considerations. 

Apparatus 100 

Mydriatics 106 

Relative advantages of plane and concave mirrors . . . 107 



INDEX 



in 



PREFACE 



THIS little book was written to bring about the more 
general adoption of Skiascopy as an essential part of 
the examination for ametropia. It is not supposed that 
any ophthalmologist is quite ignorant of the test ; but 
many do not know its full practical value, or how best to 
apply it. 

The demonstrations and descriptions here given assume 
a general knowledge of the eye and of physiological optics. 
And the writer, having observed that students of this 
subject do not generally think in the terms of algebraic 
formulas, but more readily grasp the graphic or geometric 
presentation of a fact, has governed himself accordingly. 

The claims of this subject to careful consideration are : 

First. — Skiascopy, is an objective test, independent 
of the patient's intelligence or visual acuteness, and more 
largely than any other, independent of the patient's coopera- 
tion. 

Second. — It is by far the most accurate objective 
test. The limits of its accuracy depend on details of its 
execution, and the skill and patience of the observer ; but, 
it does not require any rare natural qualifications, to carry 
it, for many eyes, to the extreme limits of accuracy for 
subjective tests. 

Third. — It requires but little more time than the use 
of the refraction ophthalmoscope or the ophthalmometer, 
which are able to give very inferior information. It saves 
time in making a complete diagnosis. 



Fourth. — It requires no costly, complex or cumber- 
some apparatus. 

Fifth. — It lays before the surgeon the refraction in 
each particular part of the pupil as it is revealed by no 
other test, and opens up the principal avenue for farther 
advance in the scientific study of the refraction of the eye. 

Of the use of the data obtained by means of skias- 
copy, it is not the purpose of the present monograph to 
speak. These data include the refraction of the visual 
zone, corresponding to the refraction of the eye obtained 
by other methods ; an accurate knowledge as to the loca- 
tion and limits of that zone ; and the refraction outside of 
it ; the latter having in some cases important bearings on 
the practical adjustment and use of lenses. 



Denver, Col., March, 1895. 



CHAPTER I. 

HISTORY, NAME, DIFFICULTIES, AND METHOD OF STUDY- 
ING THE TEST. 

History. — From the earliest use of the ophthalmoscope, 
by the direct method, it has been recognized that the see- 
ing of an erect image of the fundus at some little distance 
from the eye indicated hyperopia, and the seeing of an in- 
verted image indicated myopia. So long ago as 1862, 
Bowman {The Royal London Opthalmic Hospital Reports, 
Vol. II, p. 157), called attention to the rotation of the mir- 
ror as a means of bringing out appearances characteristic 
of irregular astigmatism and conical cornea, and Donders, 
in his work on Accommodation and Refraction of the Eye, pub- 
lished in 1864, included (p. 490) the following note : 

" My friend Bowman recently informs me that c he 
has been sometimes led to the discovery of regular astig- 
matism of the cornea, and the direction of the chief meridi- 
ans, by using the mirror of the ophthalmoscope much in 
the same way as for slight degrees of conical cornea. The 
observation is more easy if the optic disc is in the line ol 
sight and the pupil large. The mirror is to be held at two 
feet distance, and its inclination rapidly varied, so as to 
throw the light on the eye at small angles to the perpen- 
dicular, and from opposite sides in succession, in successive 
meridians. The area of the pupil then exhibits a some- 
what linear shadow in some meridians rather than in 
others.' " 

(7) 



8 SKIASCOPY. 

The use of the ophthalmoscope above referred to, for 
the detection of irregular astigmatism, became widely pop- 
ular. It was generally adopted as the most satisfactory test 
for this kind of defect. But the observation that the same 
method was capable of revealing regular astigmatism and 
the direction of its principal meridians, does not seem to 
have attracted the same attention. 

In 1872, Couper, in his paper before the Fourth Inter- 
national Ophthalmological Congress (see Trans, page 109), 
alluded to Bowman's observations, and said : " The greater 
dispersion in one meridian than in the opposite, gives rise 
to the linear shadows. Only the fact of astigmatism is 
thus established." He then went on to describe a method 
of using the ophthalmoscope as an optometer in astigma- 
tism, which is rather a modification of the ordinary use ot 
the ophthalmoscope than a variety of skiascopy, since it 
depends on the recognition or non-recognition of the retinal 
vessels in different meridians, when the ophthalmoscope 
mirror is held at a considerable distance from the eye, and 
takes no account of the movement of a light area on the 
retina. It had already been pointed out (Donder's Accom. 
and Ref. oftlie Eye, page 106), that the distance of the in- 
verted image before the eye indicated the degree of myopia. 

In 1873, Cuignet, of Lille, published (Rec. d'Ophtal- 
mol) 1873, pp. 14 and 316) an account of the test, as he 
had used it, as one capable of revealing not only the pres- 
ence of hyperopia or myopia as well as astigmatism, but 
also as giving a practical method of measuring the amount 
of these errors of refraction. He seems not to have appre- 
ciated fully the optical principles involved in the test, and 
his account of it attracted no attention. However, in 1878, 
his pupil, Mengin, introduced the practice of the method 
at Galezowski's clinic in Paris. There it was taken up, 



HISTORY. y 

and Parent demonstrated its true optical basis and urged 
its advantages in a series of articles published in the Recucil 
oV Ophlaimologie in 1 880-81, pp. 65 and 229. 

L,ytton Forbes, in the Royal London Ophthalmic Hos- 
pital Reports for 1880, (p. 62), published a paper on the test, 
giving a minute account of the various forms assumed by 
the light and shadow in the pupil, but without full expla- 
nation of their optical significance. In 1881, A. Stanford 
Morton included a full description of the test in his little 
work on the Refraction of the Eye. In 1882, Charnley gave 
the fullest demonstration of its optical basis in the Royal 
London Ophthalmic Hospital Reports, X, 3, p. 344. And Juler 
called attention to it in the Ophtlialmic Review Vol. I, p. 327. 
The method described and advocated by Parent and 
those who followed him, had been that with the concave 
mirror. Cuignet had used the plane mirror, and, in 1882, 
Chibret pointed out (Annates d' Oculisti que, Vol. xxxviii, p. 
238) the advantages of the plane mirror in determining the 
presence and degree of myopia in the examination of large 
numbers of recruits. In 1883, Story (Ophthalmic Reuieiv y 
Vol. II, page 228) advocated the use of the plane mirror,, 
but in the same manner as the concave, except that the ob~ 
server should place himself at a distance of four metres. 
from the patient, a distance which renders the test of little 
value for a considerable proportion of cases. 

In 1885, was published (American Journal of the Medical 
Sciences, April, 1885) the writer's account of the test with 
the plane mirror, as applicable to all varieties of ametropia, 
the determination being made by measuring the variable 
distance of the surgeon from the patient. Since that time 
the test has been widely recognized, but even yet is far 
from being universally adopted and depended upon as it 
deserves to be. The literature of the subject has since grown 
1 



10 SKIASCOPY. 

quite extensive. But it must be noted that a considerable 
proportion of the accounts of the test bear evidence that 
their author's acquaintance with it has been theoretical 
rather than practical, and the mass of 'them contribute 
nothing to the common fund of professional knowledge. 
The writer's contributions as to the retinal illumination 
(Ophthalmic Review, Feb., 1890) and the relative positions 
of the source of light and the observer (Archives of Ophthal- 
mology, July, 1893), with the suggestions as to the special 
pieces of apparatus to facilitate the test, to be mentioned 
in Chapter VIII, complete the evolution of this method of 
diagnosis as now practiced, and here described. 

Name of the test. — Neither Bowman nor the others, 
who early employed the test for the detection of irregular 
astigmatism and conical cornea, proposed for it any special 
name. 

Cuignet, who brought it forward as a distinct method for 
the diagnosis of the refraction of the eye, seems to have 
thought at first that the play of light and shade in the 
pupil depended entirely on the curvature of the cornea, 
and described it under the name keratoscopie. Considering 
the real causes of the movement of light and shade in the 
pupil and the purposes for which it is employed, this name 
seems especially inappropriate. 

Parent, realizing this inappropriateness, proposed retino- 
scopie, in allusion to the fact that it was the movement of 
light and shade on the pigment layer of the retina that 
commonly gave rise to the phenomena studied. Yet this 
name was obviously open to criticism, in that the condition 
of the retina itself was not at all the matter in considera- 
tion, and that the same play of light and shade could be 
watched on the head of the optic nerve, or, where the reti- 
nal pigment was wanting, upon the choroid or sclera. 



HISTORY. 11 

Chibret, to bring out the point that it was the movement 
of a shadow that was the subject of investigation, proposed 
the name of fantoscopie retinienne, and, Mr. Priestley Smith, 
probably anglicizing this term and dropping the allusion 
to the retina, called it the shadow-test. This name though 
a compound word, where a simple one should do, became 
extremely popular, and its appropriateness led Chibret to 
call to his aid, the linguistic skill of M. Egger, who ren- 
dered it in the term skiascopia, which in its French form 
skiascopie, or its English form, skiascopy, has been most 
widely accepted as the proper term to designate the test. 

Umbrascopy, proposed by Hartridge, is indefensible on 
linguistic grounds, and the same is true of pupilloscopie 
proposed by Landolt, and for which he afterwards offered 
the equivalent, koroscopie. Dioptroscopie was advocated by 
Galezowski (Atlas d' 'Ophthalmoscopic) and is appropriate, 
though equally applicable to other methods of measuring 
refraction. 

Retinophotoscopie and retinoskiascopie have have also been 
recently suggested by Parent, but there seems to be no suf- 
ficient reason for retaining in the name any allusion to the 
retina. Fundus-reflex-test suggested by Oliver is also unnec- 
essarily long for a name. 

The suggestion has sometimes been made to apply one 
of these names to one form, and another name to another 
form of the test. But such a use of them is not warranted 
by their original suggestion or by custom, nor is there any 
sufficient reason for the employment of separate names to 
differentiate various forms of the test. In all its different 
forms, the test is essentially the same ; the difference being 
merely as to the apparatus and mechanical detail. 

Difficulties of the Test. — That skiascopy, though a val- 
uable method of examination, is one difficult to master. 



12 SKIASCOPY. 

becomes more and more evident as one continues to work 
with it. The theoretical basis is perfectly simple ; the 
fundamental phenomena readily observed ; and, with a few 
days practice, the merest tyro may be able by it to estimate 
the refraction in favorable eyes with an accuracy not to be 
attained by any other objective method. But long after 
the stage of such acquirement has been passed, the surgeon 
will again and again encounter cases that still prove diffi- 
cult and puzzling. Nothing but a thorough understanding 
of the optical principles involved, and patient study of the 
eyes which prove most puzzling, under carefully arranged 
favorable conditions, will enable one to master the test. 

The importance of careful arrangement of the relative 
positions of the light and of the observer, and the adapta- 
tions of the mirror have not heretofore been sufficiently 
insisted upon. What these adaptations and arrangements 
are will appear under their proper headings in chapters III 
and IV. It is here only necessary to emphasize their 
importance. For instance : All descriptions of the shadow- 
test allude to the characteristic band-like appearance of the 
light in astigmatism. Now, as a matter of fact, even in 
the highest degrees of astigmatism, such an appearance 
cannot be perceived, except with certain lenses, or at cer- 
tain distances in front of the eye ; and it is a distinctive 
and exact indication only when the light, the mirror, and 
the patient's and the observer's eyes are brought into a 
certain relation. 

It would be as rational to attempt to measure refraction 
with an ophthalmoscope devoid of any lens series, or to test 
the acuteness of vision in a darkened room, as to expect 
definite and satisfactory results from skiascopy, applied 
without careful attention to details that have usually not 
been referred to in descriptions of the test. 



DIFFICULTIES. 13 

The fact that this test shows, as does no other, the actual 
refraction of the eye for each particular portion of the 
pupil, increases enormously the wealth of phenomena it 
offers for study, adding to its scientific and practical value, 
but also making it more difficult by rendering it necessary 
to discriminate between the particular portions of the 
movement of light and shade which are of practical im- 
portance, and others which are not. 

How to Study the Test. — The study of skiascopy is 
something quite different from its practical application. To 
start from a few bare rules as to the placing of glasses, and 
the movements of the mirror, and the light in the pupil, 
and attempt to learn the test by using it will never give a 
mastery of it. It is better to make a careful study of it 
before attempting to employ it as a method of ascertaining 
the refraction. 

Such a study is chiefly a use of the test, but from a 
standpoint entirely different from that of its application in 
practice. To study the test, one should as far as possible, 
start with known conditions of refraction, with lenses of 
known strength, with the eye at a known distance, and 
should observe the character of the movements of light and 
shadow in the pupil, which belong to these known condi- 
tions. He should work from known refraction to the 
pupillary appearances that belong to it. While in using 
the test for the measurement of ametropia, he has to deduce 
from observed pupillary appearances the state of refraction 
causing them. 

The student may, from time to time, test his progress 
towards proficiency by attempts to measure refraction by 
skiascopy, but familiarity with the appearances indicative 
of known conditions of refraction is chiefly to be sought. 

The appearances upon which the attention is fixed in 



14 SKIASCOPY. 

skiascopy are those of the red reflex in the pupil. The 
first step is to learn just what this appearance is and some 
of the variations of which it is capable. Let the beginner 
with his eye at the sight-hole of the skiascopic mirror 
throw into the observed eye, from a distance of 20 or 30 
inches, the light from a lamp flame as in the ordinary oph- 
thalmoscopic examination. Looking into the observed eye 
with the light properly directed, he will see the brilliant 
point of light, the reflection from the surface of the cornea 
of the lamp flame he is using ; and he may also see reflections 
of his own face or of other objects from the surface of the 
cornea. These are to be disregarded. The real object of 
study, the phenomena upon which attention is to be fixed, 
is the general red glow perceived within the pupil, the 
fundus reflex. 

If the mirror be rotated about an axis lying in the plane 
of the mirror, the area of light thrown by it upon the 
face will move in the direction towards which the mir- 
ror is turned. As the test becomes familiar, the direction 
of this movement will be known without any conscious 
effort to discover it. With the concave mirror at a greater 
or lesser distance than its focus or with the plane mirror at 
all distances, except at the point of reversal which it is the 
object of the test to determine, the rotation of the mirror 
also causes a movement of the red reflex in the pupil. As 
the reflex disappears from the pupil, it is followed by an 
area of shadow, and, as it returns to the pupil, the shadow 
passes out before it. The movement of the light area 
really goes on when no shadow is visible in the pupil, but 
only when light and shade are both seen can the movement 
be recognized. We know the movement of light in the 
pupil by the movement of the boundary between light and 
shade. 



STUDY OF THE TEST. 15 

Having learned what it is that he has to watch in the 
pupil, the student should make himself familiar with the 
various appearances of the fundus reflex by viewing it 
from different distances, with different lenses before the 
eye, with different mirrors, and later, if he chooses, in a 
number of different eyes ; and all this without concern- 
ing himself as to the state of their refraction, or the especial 
significance of what he does. That is, he should learn to 
some extent what are the variations in the pupillary reflex, 
a few of which are illustrated on the following pages, 
before attempting to appreciate their significance. 

Without a good understanding too of the simple optical 
principles underlying the test, it must remain a blind 
routine and rule of thumb work, and can never be of the 
highest utility. To aid in such an understanding of them, 
one may take a strong (15 D. to 20 D.) convex lens and a 
piece of card-board with a dot on it. The lens can repre- 
sent the dioptric media of the eye, the card-board the retina, 
and the dot the light area upon the retina. The card-board 
should be held back of the lens a little farther than its 
focal distance, and the dot looked at through the lens from 
various distances. Nearer the lens an erect image of the 
dot (blurred of course), and, farther away, an inverted image 
will be seen, and between the two the phenomena of rever- 
sal. The movement of light on the retina may be imitated 
by a slight movement of the card in different directions. 

The apparent enlargement of the dot, as the point of 
reversal is approached, and the diminution of its apparent 
size as the point of reversal is departed from, its diffusion 
and indistinctness near the point of reversal, and its con- 
centration and greater definiteness away from the point of 
reversal, are to be observed. Such a combination of dot and 
lens will also beautifully exhibit the phenomena of aberra- 



16 SKIASCOPY. 

tion [See Chap. V] with its central and peripheral areas of 
differing movement, the one an erect and the other an in verted 
image. The difficulty of keeping the dot in view when 
the point of reversal is approached, will illustrate how 
small a portion of the retina is visible from the point of 
reversal when the test is applied to the eye. By holding 
in combination with the spherical lens a cylindrical lens 
of 5 D., the distortions of the fundus reflex produced by 
astigmatism, and the band-like appearances it causes at 
certain distances, should also be studied. 

This is not all to be done at a single trial, but the lens 
and card should be kept at hand where they can be used to 
parallel and elucidate the different conditions as they arise in 
studying the pupillary reflex. 

The study of the appearances in the eye may thus be 
carried on : Take an eye, the refraction of which is known, 
and from a distance that will give an erect movement, 
throw the light into the eye, and, by the rotation of the 
mirror, produce and study the erect movement. Then with 
a lens which it is known will give an inverted movement, 
the inverted movement is to be similarly studied. Finally 
the lens, or position of the observer is to be so varied as to 
bring the point of reversal to the eye, and the appearance 
of the pupil from this point is also to be studied. In these 
studies, and, indeed, throughout the whole course, the 
student will find it easier to master and understand first 
the appearances with the plane mirror. 

If it is possible to get an eye free from astigmatism or 
aberration of any notable degree, these earlier studies of 
the appearances will be much simplified. After the above 
have become familiar, the phenomena of astigmatism may 
be studied by placing before the same eye, a cylindrical 
lens of known strength. The point of reversal with such 



STUDY OF THE TEST. 17 

a lens will give an observer the appearances presented by 
the pupil at this distance and at the other distances the 
other appearances presented in astigmatism can be ob- 
tained. 

For example, suppose the eye at the student's disposal is 
hyperopic i D. Let him first place before it the convex 
2 D lens. This will bring the point of reversal one metre 
from the eye. With the plane mirror, let him first study 
the erect movement at one-half metre ; then study the 
inverted movement at a distance of two metres ; then 
observe the eye from the point of reversal at one metre, and 
then vary his distance so as to study it from intermediate 
points. 

When he takes up the study of astigmatism, he should 
place before such an eye, a convex cylindrical lens of 2D 
in addition to the spherical. Then from the distance of 
one-third of a metre he will be able to observe the band of 
light at right angles to the axis of the lens, from a distance 
of one metre the band of light running in the direction of 
the axis of the lens, and from other distances the other 
appearances indicative of astigmatism. 

Familiarity with the many appearances due to aberra- 
tion and irregular astigmatism is only to be obtained by 
study of eyes presenting those defects. But, as the great 
majority of eyes present them in notable degree, material 
for such a study is not difficult to obtain. Careful observa- 
tions of the corresponding appearances, with the lens and 
card-board already referred to, will enable the beginner 
promptly to recognize the appearances of aberration. And, 
when once he has found an eye that presents them, let him 
observe them with different lenses, and [with the plane 
mirror] from varying distances. 

A considerable part of the study of skiascopy and espe- 



18 SKIASCOPY. 

cially of the appearances of positive aberration can be 
carried on with the aid of an artificial, schematic, or model 
eye. That of Frost is one of the best, although any, even 
the rudest, will answer. In the studies on the human eye, 
it is better to study one eye long and repeatedly, or at most 
to confine the earlier observations to a few eyes than to 
attempt to employ a large number. Each additional eye 
will introduce variations in the appearances presented, 
which will at first be only^puzzling and retard, rather than 
assist, the mastery of the test. 



CHAPTER II. 

GENERAL OPTICAL PRINCIPLES. MYOPIA, EMMETROPIA, 
HYPEROPIA. 

Skiascopy is a method of measuring myopia, either the 
myopia originally present in the eye or that produced by a 
lens of known strength for the purpose of measurement. 
In myopia, we have the retina situated back of the princi- 
pal focus of the dioptric media, so that the rays of a certain- 
divergence, that is coming from a point a certain finite dis- 
tance in front of the eye, are brought to a focus upon the 
retina. Conversely, the rays coming from a point on the 
retina and passing out through the crystalline lens and 
cornea, are brought to a focus at the same distance in front 
of the eye. The point for which the eye is focused, and 
the point on the retina, on which the focused rays are 
received, have the relation of conjugate foci to the refract- 
ive surfaces of the eye. 

The Reversal of Movement. — The amount of myopia 
is known when we know the distance of the point in front 
of the eye, which has this relation of a focus conjugate to 
the retina. Skiascopy furnishes a method of determining 
the position of this point. Closer to the eye, than this 
point for which it is focused, the observer may see an erect 
image of the fundus. Farther from the eye than this point, 
he can perceive an inverted image. Skiascopy is a means 
of determining when the image seen is erect and when it 
is inverted, or when it passes from the erect to the inverted. 

(19) 



20 GENERAL OPTICAL PRINCIPLES. 

This may be understood from a study of figure I. 
Let M represent a myopic eye, A and B being two points 
of the retina from which rays emerge to reach the ob- 
server's eye ; and C and D the points at which these rays 
coming from the retina are focused, the rays coming from 
A being focused at C and those from B at D. 

The apparent position of a point is determined by the 
direction of a ray coming from that point and passing 
through the nodal point of the observer's eye. Suppose 
the observer's eye is placed at N, closer than the point for 
which the observed eye is focused. The apparent position 
of the point A is determined by a ray which passes through 
the upper part of the pupil and is turned down. It appears 



- -*. — — - fi 

Fig. i. 

in the direction of a. The apparent position of the point 
B will be located by the ray coming through the lower 
part of the pupil and turned up. It will be seen in the 
direction of b. Thus, from this position N, the point A, 
which is really above appears above, and the point B, 
which is really below appears below. The observer sees an 
erect image. 

When, however, the observer places his eye at N', at 
a greater distance than that for which the eye is focused, 
the ray which reaches his nodal point from A, will be 
one that comes through the lower part of the pupil and is 
turned up ; so that A will appear to be located in the 
direction of a' in the lower part of the pupil. From this 



REVERSAL OF MOVEMENT. 21 

position he will judge the location of B by the ray which 
comes through the upper part of the pupil and is turned 
down, so that B will appear to be located in the direction 
of b' in the upper part of the pupil. That is, the point A, 
which is really above, will appear to be below, and the 
point B, which is really below will appear to be above. 
The image observed is inverted. 

The Point of Reversal. — It is evident that this change 
in the relation of the rays that brings about the change in 
the apparent position of A and B occurs at the distance of 
the points C and D, at which, the rays coming from the 
retina are focused. Here it is that these rays intersect and 
take their new relation which gives the reversal of the 
apparent position of the points of the retina from which 
they come. 

It is, therefore, convenient in connection with skiascopy 
to designate this point as the point of reversal. This name 
indicates the significance of this point with reference to 
this test. Of course, it is really the same point as the far 
point of the myopic eye — the point for which the eye is 
focused — the conjugate focus of the retina — these latter 
names indicating the relations of the same point in other 
matters. 

It is only when the rays leave the eye, convergent only 
when the eye is myopic, that they ever come to a focus in 
front of it. If the eye be emmetropic or hyperopic, the 
rays emerging parallel or divergent remain so at all dis- 
tances. Hence, in emmetropia and hyperopia, there can be 
no point of reversal. From whatever distance the eye is 
viewed, the image perceived is erect. 

In myopia, the distance of the point of reversal from 
the eye depends on the degree of convergence of the rays 
as they leave the cornea — depends on the amount of myo- 



22 GENERAL OPTICAL PRINCIPLES. 

pia. The distance of the point of reversal from the eye 
being the distance from the eye to its far point is the focal 
distance of the lens required to correct the myopia. So 
that to ascertain the amount of myopia, we have only to 
determine the point of reversal and then measure its dis- 
tance from the eye. 

Skiascopy determines the position of the point of re- 
versal by observation of the direction of the movement of 
light and shade in the pupil. Other kinds of ophthalmo- 
scopic examinations attempt the recognition of the details 
of the fundus image. But, as the point of reversal is 
approached, the details of the fundus image become indis- 
tinct and fade away entirely, so that the location of the 
point of reversal cannot be accurately determined by such 
an examination. On the other hand, when this point has 
been so closely approached that the fundus details are quite 
indistinguishable, it still remains easy to recognize the 
direction of the movement of light and shade in the pupil ; 
and, from it, to deduce the erect or reversed character of 
the image. Skiascopy, therefore, determines the point of 
reversal, and measures the degree of myopia with much 
greater exactness than the fundus-image tests. 

In skiascopy, we watch the apparent movement of light 
and shade in the pupil, due to the real movement of an 
area of light upon the retina. This area of light is secured 
by reflecting into the eye the light from a lamp with a 
skiascopic mirror. This is done in a darkened room, in 
order that the retina outside of this light area may be dark, 
furnishing a contrast for the movements to be watched. 
The movement of the light area upon the darkened retina 
is secured by varying the inclination of the mirror — rotating 
it about some axis passing through the sight hole. The 
movement produced by a certain change in the position of 
the mirror depends on whether it is plane or concave. 



MOVEMENT OF LIGHT ON THE RETINA. 23 

Real Movement of the Light on the Retina. The 
Source of Light. — The lamp flame, or similar source of 
light used for the test, may be called the original source of 
light, in contra-distinction to the reflection of it from the 
mirror, which being more immediately related to the move- 
ment of the light on the retina, we shall call the immediate 
■source of light. 

The Plane Mirror. — With the plane mirror the imme- 
diate source of light is behind the mirror as far as the 
original source of light is in front of it. The rays reflected 
from the mirror enter the eye under observation as though 
they had started from this immediate source. As the mir- 
ror is rotated, the apparent position of the immediate 
source of light changes ; for this immediate source is sit- 
uated upon a line drawn through the original source per- 
pendicular to the surface of the mirror, and necessarily 
changes with that perpendicular as the inclination of the 
mirror changes. 

With the change of position of the immediate source 
of light, the rays coming from it and falling upon the eye, 
are made to fall upon a new part of retina, and thus the 
inclination of the mirror causes the change in the part of 
the retina that is lit up by the light reflected into the eye. 

A 

i 




\ B 



A 

Fig 



What these changes are can be better understood by a 
study of figure 2. L represents the position of the lamp 
flame, the original source of light. When the mirror is 



24 GENERAL OPTICAL PRINCIPLES. 

held in the position AA, the immediate source of light is 
situated at 1, and light entering the eye from that direction 
falls upon the retina toward a. When, however, the posi- 
tion of the mirror is changed to BB, the immediate source 
of light is changed to 1', from which, light falls upon the 
retina towarb b. As the mirror is rotated from A A to BB, 
the position of the immediate source of light moves from 1 
to 1', and, as a consequence, the area of light upon the 
retina moves from a to b. The light on the retina then, 
moves in the direction that the mirror is made to face. It 
is said to move with the mirror. 

Only a portion of the light reflected by the mirror 
enters the eye, the remainder falls upon the face and makes 
a light area on the face. One may readily demonstrate by 
trial that this area of light cast by the mirror on the face 
also moves with the mirror under all circumstances. 

The rays of light coming from 1 and V intersect at the 
nodal point of the eye ; and passing directly on do not again 
change their relative position. Whatever the distance of 
the retina from this nodal point, the movement of the light 
upon it will be in the same direction, so that whether the 
retina be at H. as in hyperopia, at E. as in emmetropia, or 
at M. as in myopia, the real movement of light upon it 
from a certain movement of the mirror is always in the 
same direction. 

Therefore, with the plane mirror, the real movement of the 
area of light on the retina is with the mirror — with the area of 
light on the face, in all states of refraction. This is true for 
all distances of the light from the mirror, or of the light 
and mirror from the tested eye. 

The Concave Mirror. — W T ith the concave mirror as 
used in skiascopy, the immediate source of light is a real 
focus of the mirror, conjugate to the position of the light, 



MOVEMENT OF LIGHT ON THE RETINA. 



25 



and is usually situated between the mirror and the eye to 
be tested. The position of this immediate source varies 
with the position of the mirror, moving in the direction 
that the mirror is made to face and causing an opposite 
movement in the area of light that falls from it upon the 
retina. 



„^_^~~^ 




Fig. 3. 

In figure 3 L again represents the original source 
of light. When the mirror is in the position AA, the 
light falling upon it from L, is focused at 1., and the 
little inverted image of the lamp flame there formed is the 
immediate source of light. From it the rays diverge, some 
to fall upon the face, and those entering the eye to fall 
upon the retina toward a. When the mirror is turned to 
occupy the position BB, the light falling upon it is focused 
at 1', which becomes the new position of the immediate 
source of light, and from which the rays entering the eye 
fall upon the retina toward b. As the mirror is rotated 
from AA to BB, the immediate source of light moves from 
1 to V and the light upon the retina from a to b. This will 
be the direction of its movement in all states of refraction 
whether the retina be situated at H. as in hyperopia, at E. 
as in emmetropia, or at M. as in myopia. The portion of 
the light which falls upon the face, however, and forms the 
facial area, as can be readily demonstrated by trial, moves 
in the direction that the mirror is made to face. 



26 GENERAL OPTICAL PRINCIPLES. 

We have then, with the concave mirror, the real movement 
of the area of light on the retina is against the mirror, and 
against the light on the face, in all states of refraction. 

The above is the movement that occurs with the con- 
cave mirror used as in skiascopy, so far from the original 
source of light and from the eye to be tested, that the con- 
jugate focus of the original source of light falls in front of 
the eye. If, however, the original source of light be 
brought so close to the mirror that the rays from it are not 
rendered convergent, but continue to diverge after reflec- 
tion, the immediate source of light will be a magnified 
image of the lamp flame, situated behind the mirror as in 
the case of the plane mirror ; and the movement of the 
retinal light area will be precisely the same as with the 
plane mirror. Again, if the rays reflected by the mirror 
are rendered convergent, but the eye to be tested is brought 
so near that they cannot come to a focus in front of its 
nodal point, the light will pass in as though from an imme- 
diate source back of the mirror, and the movement of the 
area of light on the retina will again be like that with the 
plane mirror. If the light reflected upon the eye be con- 
vergent so as to be focused just at its nodal point, no move- 
ment of light on the retina such as we have been consider- 
ing will occur, but whatever direction the mirror is turned, 
so long as the light enters the eye, the retinal light area 
remains stationary. 

It is to be borne clearly in mind that the movement so 
far spoken of is the real movement of the area of light upon 
the retina as it would appear from within the eye itself, or 
when viewed from behind the retina with the sclera and 
choroid cut away. 

The Apparent Movement of the Light in the Pupil. 
— What we observe in skiascopy, however, is the apparent 



APPARENT MOVEMENT IN THE PUPIL. 27 

movement of the light in the pupil as viewed from the 
position of the observer some distance in front of the eye. 
When an erect image of the retina is viewed, this apparent 
movement of the light will be in the same direction as the 
real movement. When an inverted image is viewed, the 
apparent movement will be in the direction opposite to that 
of the real movement. 

The observer can always w r atch the movement of the 
light area on the face, and know that with the plane mir- 
ror the light area on the retina always has a real movement 
in the same direction, and with the concave mirror it always 
has a real movement in the opposite direction ; and he has 
only to compare the apparent movement of the light which 
he watches in the pupil with the known direction of the 
real movement on the retina, to determine whether he sees 
an erect or an inverted image, When the apparent and 
real movements are in the same direction, he knows (page 
19) he is looking at the eye from a distance shorter than 
that for which it is focused. When the apparent and real 
movements are in opposite directions, he knows that he is 
looking at the eye from a distance greater than that for 
which it is focused. 

The direction of the apparent movement of the light 
then, will be with the light on the face in hyperopia and 
in emmetropia at all distances, and in myopia when the 
eye is viewed from a point nearer than its point of reversal, 
and the apparent movement in the pupil will be the opposite 
of the real movement only in cases of myopia when the 
eye is viewed from somewhere beyond its point of reversal. 

With the plane mirror, the apparent movement will be with 
the light on the face in hyperopia, emmetropia, and myopia with 
the point of reversal behind the observer; and against the light 
on the face in myopia viewed from beyond the point of reversal. 



28 GENERAL OPTICAL PRINCIPLES. 

With the concave mirror the apparent movement is against 
the light on the face in hyperopia, emmetropia, and myopia with 
the point of reversal behind the observer ; and is with the light 
on the face only in myopia viewed from beyond the point of 
reversal. This statement made to conform to the practice 
customary in the use of the concave mirror where the 
observer keeps a constant distance of i metre from the eye 
[corresponding to i D. of myopia] would be : the light 
moves against the light on the face and against the mirror in 
hyperopia, emmetropia and myopia of less than 1 D., and only 
moves with the light on the face in myopia of more than 1 D. 

These statements are made with reference to the 
apparent movement of the light before the state of refrac- 
tion has been modified by any glass placed before the eye 
for that purpose. But the}- hold equally as to hyperopia,, 
emmetropia, or myopia remaining uncorrected, or produced 
by a lens placed before the eye. For instance : — In myopia 
the movement remains against the light on the face with 
the plane mirror, or with the light on the face with the 
concave mirror, so long as the concave lens employed is 
not strong enough to bring the point of reversal to the dis- 
tance of the observer's eye. In hyperopia or emmetropia, 
where the movement is watched through a convex lens, 
the movement remains with the light on the face for the 
plane mirror, and against the light on the face for the con- 
cave mirror, until a convex lens is used strong enough to 
over-correct the hyperopia and cause enough myopia to 
bring the point of reversal nearer to the eye than the posi- 
tion of the observer. 

Rapidity of Movement of the Light on the Retina. 
— The rapidity with which the light and shadow appear to 
move across the pupil depends first, on the rapidity of the 
real movement of the light area upon the retina ; and v 



RAPIDITY OF MOVEMENT. 29 

second, upon the magnification of the retina. The rapidity 
of the real movement on the retina depends : 

On the rate of movement of the mirror in the observ- 
er's hand. 

On the distance of the mirror from the observed eye. 

On the distance of the original source of light from 
the mirror. 

And upon the distance of the retina from the nodal 
point of the eye. 

The rate of movement of the mirror and the distance 
of the light from the mirror determine the rapidity of the 
movement of the immediate source of light ; this being 
greater as the mirror is moved more quickly, or as the ori- 
ginal source of light is more distant from the mirror. The 
excursion which the immediate source of light can make 
is limited by the width of the mirror, and the extent of 
movement of the light area on the retina produced by the 
movement of the immediate source of light entirely across 
the mirror depends on the relative distance of the mirror 
and the retina from the nodal point of the eye. The wider 
the mirror, or, the nearer it is to the nodal point of the eye, 
or the farther the retina is from that nodal point, the 
greater the extent of movement produced in the retinal 
area of light by a given movement of the mirror. On 
account of the relative distances of the retina from the 
nodal point, the extent of the movement of the light on 
the retina is, other things being equal, least in the highest 
hyperopia and greatest in the highest myopia. 

The rapidity of the real movement of the light on the 
retina then, is increased : 

By moving the mirror faster. 

By carrying the original source of light farther from 
the mirror. 



30 GENERAL OPTICAL PRINCIPLES. 

By bringing the mirror closer to the eye. 

By elongation of the antero-posterior axis of the eye- 
ball. 

The real movement of the light upon the retina is 
made slower : 

By moving the mirror more slowly. 

By bringing the original source of light closer to the 
mirror. 

By carrying the mirror farther from the eye. 

By shortening of the antero-posterior axis of the eye- 
ball. 

In using the test, the distance of the light from the 
mirror is practically constant, and the ordinary variations 
in the antero-posterior axis of the eye-ball are so slight as 
to have no appreciable influence. So that the rapidity of 
the real movement of light on the retina depends princi- 
pally on the rapidity of the movement of the mirror and 
the distance of the mirror from the eye. 

Magnification of the Retina. — In practice the rapidity 
of the apparent movement of the light in the pupil depends 
far more on the extent to which the retina and the real move- 
ment of light upon it are magnified, than upon the actual 
rate of that real movement. The retina as viewed through 
the pupil from different distances, is seen under different 
degrees of magnification. When the observer's eye is 
placed at the point of reversal, the rays from a single point 
of the retina, passing through all parts of the pupil, con- 
verge to the observer's nodal point, so that the one point of 
the retina appears to occupy the whole of the pupil, and 
the retina is seen indefinitely magnified. As the observer's 
eye departs from the point of reversal, it receives the rays 
from an increasing area of the retina, more and more of 
the retinal image occupies the same space of the pupil and 



MAGNIFICATION OF THE RETINA. 31 

the retina is seen less magnified. 

This is illustrated in figure 4, which represents an 
eye with its point of reversal at A. If the observer's eye 
be placed at A it receives rays only from the point a, and 
this point appears to occupy the whole pupil. If, however, 
the observer's eye be placed at B, from which rays would 
be focused at b behind the retina, and, at which, rays from 
b would be focused, the observer will be able to see in 
the space of the pupil all of the retina, m n, included 
between the broken lines passing from B to b — all of the 
retina, which would receive a circle of diffusion if the rays 
were coming from the point B. Or, again, if the observer's 



Fig. 4. 

eye be placed at C, from which rays will be focused at c in 
front of the retina, and, at which, rays coming from c would 
be focused, he will be able to perceive the portion of the 
retina, m n included, between the dotted lines, passing 
through c and continued on to the retina — the area upon 
which would be formed a circle of diffusion by rays coming 
from the point C. 

It follows then, that the closer the observer's eye to 
the point of reversal, the more is the real movement of 
light upon the retina magnified, and, therefore, the swifter 
does it appear. The farther the observer's eye is removed 
from the point of reversal, the less is that real movement 
of the light on the retina magnified ; and the slower is the 
apparent movement as watched in the pupil. 



32 GENERAL OPTICAL PRINCIPLES. 

And, as this source of variation overcomes all other 
sources of variation in the rate of the apparent movement 
of the light, [except the rate of movement of the mirror, 
which is to a considerable extent under the control of the 
observer] the rapidity of the apparent movement of light and 
shade in the pupil increases as the point of reversal is approached 
and diminishes as that point is departed from, and constitutes 
a measure of the degree of ametropia remaining uncor- 
rected. 

Form of the Light Area. — The real form of the light 
area on the retina, except under certain conditions in 
astigmatic eyes, will be circular. If the light be perfectly 
focused on the retina it is circular, because that is the form 
of the source of light employed (see Chapter III). If the 
light be not perfectly focused on the retina, the circular 
pupil gives its form to the resulting area of diffusion. 





Fig. 5. Fig. 6. 

The apparent form of the light area as seen in the pupil 
of astigmatic eyes will be discussed in Chapters IV and V. 
But in eyes free from astigmatism this form varies with 
departure of the observer's eye from the point of reversal. 
If the magnification of the retina is so slight that all of it 
occupied by the light area is visible in the pupil at one 
time, that area appears circular as represented in figure 5. 
But when the point of reversal is approached so that the 
magnification of the retina prevents all of the retinal light 



FORM OF THE LIGHT AREA. 33 

area from being seen at one time, only a portion of its out- 
line is visible as an arc of the greatly enlarged circle, as 
shown in figure 6 ; and the nearer to the point of reversal 
that the observer comes, the nearer does the boundary be- 
tween light and shade approach to a straight line. It must 
be borne in mind, however, that this is still part oi the 
boundary of a circle, and hence that different parts will run 
in all the different directions ; in contradistinction to the 
band-like appearance of astigmatism, the direction of which 
always conforms to one or the other of the principal meri- 
dians (see pp. 47, 55). 

Brilliancy of the Light in the Pupil. — This depends 
on the illumination of the retinal light area and the extent 
to which that area is magnified. 

The illumination of the light area on the retina 
depends on the brightness of the original source of light 
and the accuracy with which the light coming from it is 
focused on the retina. The brighter the source of light 
and the more accurately it is focused, the brighter the illu- 
mination of the retiua. The dimmer the light and the 
larger the circle of diffusion over which it is dispersed, the: 
more feeble the retinal illumination. 

As the immediate source of light is usually near the 
mirror (in front for the concave, behind for the plane) 
when the mirror and the observer's eye approach the point 
of reversal, or the point of reversal is brought to them by a 
change of lenses, the light being more nearly focused on 
the retina, the retinal illumination becomes brighter. 

But, as the point of reversal is approached, the appar- 
ent brightness of the light area in the pupil is diminished 
by the increasing magnification of the retina, which causes 
the light from a smaller part of the retinal area to occupy 
the whole space of the pupil. Hence the brightest light 



34 GENERAL OPTICAL PRINCIPLES. 

reflex is never obtained at the point of reversal, but usually 
in practice at one or two dioptres from the point of reversal, 
its exact position being dependent on the arrangement of 
the source of light. 

Finding the Point of Reversal. — The point of reversal 
is to be recognized only when the observer's eye is in its 
immediate neighborhood. This may be effected either by 
varying the distance of the observer's eye from the observed 
eye until it comes to the position of the point of reversal, 
or by varying the position of the point of reversal by 
changes in the lenses placed before the observed eye until 
the point of reversal comes to the chosen position of the 
observer's eye. For reasons to be stated in Chapter VI, the 
former method is the better when using the plane mirror, 
and the latter is to be resorted to when the concave mirror 
is employed. In any case, the trial movement across the 
pupil shows by the direction of the movement whether a 
point of reversal exists between the observer and the 
observed eye, and the rapidity of movement shows approx- 
imately [when the observer has learned to appreciate its 
significance] the extent of the interval between the position 
of the observer and the point of reversal. If the movement 
be slow, the interval may amount to several dioptres. If it 
be rapid, the interval is less. 

Upon these data of the direction and rapidity of the 
movement, the surgeon bases the next step of the test, the 
selection and placing of the lenses before the eye. This 
being accomplished, the test is repeated, the movement 
with the lens noted both as to its direction and rapidity, 
and the distance of the observer from the patient, or the 
strength of the lens before the observed eye, varied in 
accordance therewith. This process is continued until the 
observer's eye reaches the point of reversal, or the point of 



POINT OF REVERSAL. 35 

reversal is brought by the lens to the observer's eye. But 
the test should not be regarded as completed until the 
movement has been repeatedly viewed both from within 
and beyond the point of reversal, as well as from that point. 
Only by this precaution of observing from a slightly greater 
and a slightly less distance, or with a slightly stronger or 
slightly weaker lens than that which brings the point of 
reversal to the surgeon's eye, can the certainty of a correct 
result be assured. 



CHAPTER III. 

CONDITIONS OF ACCURACY. 

Since in skiascopy one has to observe the movement 
of an area of light across the shaded retina, the size, bright- 
ness and sharpness of the contrast between the margin of 
this light area, and the shadow immediately adjoining it 
are very important factors in determining the defmiteness 
and accuracy of the test. For reasons to be presently dis- 
cussed, the contrast between light and shadow as seen in 
the pupil necessarily diminishes as the point of reversal is 
approached. It is, therefore, important to have the contrast 
between light and shadow upon the retina as sharp as pos- 
sible. 

Darkening the Room. — To secure this contrast, the 
retina outside of the proper light area should be in absolute 
darkness. This requires a complete darkening of the room 
in which skiascopy is practiced, including the shading of 
the source light, except in the direction in which it is 
used. The difference in the ease of the test as applied 
in a completely darkened room, as contrasted with its use 
in a partially darkened room, can only be appreciated by 
one accustomed to applying it under the former condition. 

The Source of Light. — To secure the brilliant illumi- 
nation of the light area, in contrast with the complete 
shadow around it, the source of light must be as bright as 
possible. On account of the difficulty about the sight hole 
to be referred to later, the arc electric light cannot be 

(36) 



THE SOURCE OF LIGHT. 37 

"employed, except to illuminate a piece of ground glass as 
suggested by Derby. The incandescent electric light is 
not available on account of its form, so that recourse must 
be had to one of the various illuminating flames. Of these 
the paraffin candle is the most brilliant. Next come the 
heavy mineral oils, and gas flames reinforced with the 
richer hydro-carbons, or used on the Welsbach mantle, and 
after this the ordinary illuminating gas. But a good flame 
of the latter furnishes a satisfactory illumination. 

It is more important whatever flame is used that the 
brightest part of it should be employed. With all flames 
there is at the margin a comparatively gradual shading 
from light to darkness, which interferes with the sharpness 
of the boundary of the light area on the retina. To secure 
that sharp boundary as well as to prevent the diffused illu- 
mination of the room, and to limit the size of the source of 
light, the flame should be entirely covered by an opaque 
shade with an aperture of the proper size placed opposite 
the most brilliant part of the flame. This gives, under 
proper conditions of focusing, a perfectly sharp margin to 
the light area on the retina. 

The size of this opening in the opaque screen deter- 
mining the size of the original source of light is governed 
by various conflicting requirements. Enough light must 
be available to give a distinct area of light upon the face, 
as well as to give sufficient illumination within the pupil. 
The source of light must be considerably larger than the 
sight hole in the mirror. As the mirror is rotated, the 
immediate source of light appears to move across it, and if 
this source were not larger than the sight hole, it would, 
at times, entirely disappear within that opening. At such 
times, the light area would disappear from the retina and 
from the pupil, causing delay and uncertainty in the test. 



38 CONDITIONS OF ACCURACY. 

If, however, the immediate source of light is sufficiently 
larger than the sight hole, no such disappearance of the 
light occurs. 

On the other hand, the ease of distinguishing special 
forms of the light area, or the different movements in the 
different parts of the pupil is proportioned to the smallness 
of the source of light used. The characteristic band-like 
appearance of astigmatism is developed in proportion as 
the light area upon the retina approaches the limit of a 
mathematical point. 

The size of the opening through which light is ob- 
tained, is then a compromise between the requirements of 
light and the size of the sight hole on the one hand, and 
need to have the retinal light area as small as possible on 
the other. In practice the writer prefers an aperture five 
millimetres in diameter, but the beginner may find one of 
double that diameter more satisfactory. 

Focusing of the Light on the Retina. — When the 
rays coming from the immediate source of light are accu- 
rately focused upon the retina, the area of retinal illumina- 
tion will be the smallest and brightest, and will have the 
most definite edge. This accurate focusing is secured only 
when the immediate source of light is situated at the point 
of reversal. In searching for the point of reversal, it is, 
therefore, advantageous to keep the immediate source of 
light as close to the mirror as possible. 

With the plane mirror the immediate source of light 
being a reflection of the original source as far behind the 
mirror as the immediate source is in front of it, the closer 
the original source of light can be brought to the mirror, 
the closer will its reflection be to the observer's eye, and to 
the point of reversal at the critical moment when the 
observer's eye reaches that point. The original source of 



FOCUSING OF THE LIGHT ON THE RETINA. 39 

light then should be kept as close to the mirror as possible 
being moveable to follow the movements of the observer's 
eye and the mirror when the distance of these from the eye 
under observation is varied. 

When the observer withdraws to the distance of two 
metres or more from the patient, it may not be practicable 
to keep the light very close to the mirror, but at such a 
distance, the separation of the source of light from the 
mirror becomes of small importance. For, if the original 
and immediate sources of light were at the mirror, the rays 
from the latter would have a divergence of one-half D. 
when they reached the eye ; and, if the original source of 
light were a metre in front of the mirror, so that the imme- 
diate source would be one metre behind the mirror, that is 
three metres from the eye, the rays from it would reach 
the eye one-third D. divergent, and the difference between 
the one-half and the one-third D. is so trifling as to be of 
no practical importance. 

On the other hand, when the surgeon approaches close 
to the patient's face, the slight distance that must necessa- 
rily remain between the original source of light and the 
mirror becomes a source of imperfect focusing of the light 
on the retina ; and, therefore, of inexactness in the deter- 
mination of the point of reversal. Suppose the mirror to 
be at five inches from the eye and the original source of 
light three inches from it, this will make the immediate 
source of light eight inches from the eye, and the rays from 
it will reach the pupil 5 D. divergent when the surgeon is 
seeking the point of reversal corresponding to 8 D. of myo- 
pia. This difference of 3 D. interferes greatly with the 
delicacy of the test. 

With the concave mirror, the immediate source of light 
being a real image of the original source in front of the 



40 CONDITIONS OF ACCURACY. 

mirror, cannot be brought closer to the mirror than its 
principal focal distance. It is brought closest by carrying 
the original source of light as far away from the mirror as 
possible. The original source of light then, for the con- 
cave mirror, should be behind the patient as far as possi- 
ble. 

Position of Greatest Accuracy. — With the plane mir- 
ror the immediate source of light is necessarily behind the 
mirror. It will, therefore, be exactly at the point of rever- 
sal when the mirror and the observer's eye are slightly 
within the point of reversal. Hence the conditions of 
accuracy are better complied with for the observation that 
is made from within the point of reversal, where the light 
still moves in the pupil with the light on the face, than for 
the observation that is made from beyond the point of 
reversal, where movement is inverted. The point of rever- 
sal is then, with the plane mirror, most closely approxima- 
ted from the side toward the observed eye ; and in practice 
the greatest accurrcy is attained by considering the point 
of reversal as located at the greatest distance from the eye 
at which erect movement can be seen in the visual zone of 
the pupil. 

With the concave mirror the immediate source of light 
being necessarily in front of the mirror, can be brought 
accurately to the point of reversal only when that point of 
reversal is the focal distance of the mirror in front of the 
observer's eye. It is approached more accurately by the 
lens which still leaves it in front of the observer's eye, 
than by the lens which removes it back of the observer's 
eye. Hence, with the concave mirror, the strongest con- 
cave lens, or the weakest convex, which allows the move- 
ment of light in the pupil with the light on the face, is 
the lens which brings the point of reversal most accurately 
to the distance chosen. 



POSITION OF GREATEST ACCURACY. 41 

In regular astigmatism, as will be indicated in the 
chapter (IV) upon that subject, the arrangement of the 
light must be modified, when it is desired to develop the 
band-like appearance characteristic of that condition. For 
the measurement of refraction in either of the principal 
meridians, the adjustment of the light should be precisely 
the same as for simple hyperopia or myopia. But the band- 
like appearance cannot certainly be recognized unless the 
necessary conditions as to the position of the observer and 
source of light are carefully observed. When the proper 
precautions are taken one can get a characteristic band of 
light with even one-half dioptre of astigmatism, and by 
that band can fix the direction of the principal meridians 
with great accuracy. 

In the higher degrees of regular astigmatism there is 
•considerable difficulty in measuring the refraction of the 
principal meridians with accuracy. It is, therefore best, 
before regarding the skiascopic test as completed to place 
before the eye such a cylindrical lens as appears to be 
required to correct the astigmatism, repeat the test, and so 
ascertain whether the astigmatism has been accurately cor- 
rected. Fuller references to this matter will be found in 
the Chapters VI and VII. 

Irregularities of the Media and Surfaces. — These in- 
terfere with skiascopy not only by changes in the apparent 
movement of the light as watched in the pupil, but also by 
preventing the perfect focusing of the light which falls 
upon the retina, and in this way, they limit to some extent, 
the accuracy of the test, since they are present in some 
degree in nearly all eyes. 

In the case of positive aberration (see Chapter V), the 
interference with focusing is of the same kind as the defect 
in the refraction of a strong convex spherical lens. If one 



42 CONDITIONS OF ACCURACY. 

takes such a lens and intersects the narrowing pencil of 
rays that have passed through the lens with a piece of card 
board, he will find that the strong refraction at the margin 
of the lens causes a ring of condensation at the periphery 
of the circle of diffusion. This ring is exhibited from 
close behind the lens back to its principal focus, beyond 
which, we have the condensation at the centre of the light 
area and a gradual fading away of light around it. Hence, 
the circle of diffusion in front of the principal focus pre- 
sents a brilliantly illuminated edge in sharp contrast with 
the shadow around it, while at the principal focus and be- 
hind it, the light area has a sharply defined edge, but fades 
gradually into the shadow around it. 

Therefore, in making the test, the influence of posi- 
tive aberration upon the distribution of light in the light 
area is to be utilized by having the light focused not exactly 
on the retina, but slightly back of it. This may be brought 
about by having the immediate source of light closer to 
the eye than the observer's eye or the point of reversal ; 
conditions that are secured in the use of the concave mir- 
ror. Hence, for positive aberration of a certain distribu- 
tion in the pupil, a sharper and more definitely bounded 
light area is to be obtained by the use of the concave mir- 
ror than can be had with the plane mirror. 

With negative aberration, where the refraction is 
weaker near the periphery of the pupil, the condensation 
ring of light is less pronounced and is found back of the 
principal focus for the central visual area. For this form 
of aberration the plane mirror enabling the observer by 
pushing the source of light from the mirror to get the light 
focused in front of the retina has some advantage over the 
concave mirror. 

The interference with the focusing of the light on the 



IRREGULARITIES OF THE MEDIA AND SURFACES. 43 

retina due to irregular astigmatism cannot be overcome in 
any way, and it impairs the value of the test and makes it 
more difficult to apply in eyes presenting marked defects 
of this kind. 

Distance of the Surgeon from the Patient — It will 
always be impossible to determine the point of reversal 
with perfect exactness. The best that can be done is to 
make out that it lies, within narrow limits of possible 
error, at about a certain distance. It may be an inch or 
two nearer, it may be an inch or two farther off. 

If the distance be a short one, if the lens used is such 
that the point of reversal is brought close to the observed 
eye, the possible inaccuracy of distance will cause an 
appreciable error in estimating the refraction measured in 
dioptres. For instance : at eight inches from the eye, two 
inches additional, making ten inches, will mean a whole 
dioptre of refraction, and two inches less, making the dis- 
tance six inches, will mean a difference of a dioptre and a 
half. On the other hand, at eighty inches, a foot either 
way will correspond to less than one-quarter of a dioptre of 
inexactness. Hence, for accurate work it is best to make 
the determination of the point of reversal at the greatest 
distance at which it can be certainly made in the visual 
zone. The importance of this has been especially dwelt 
upon by Randall (Trans. Section on Ophthalmol. Am Med. 
Assoc, 1894, p. 63). 

What this distance may be will vary in different eyes. 
In general, it is limited by the size of the area in which 
the movement of light and shade is to be watched. The 
pupil fully dilated may be eight or ten millimetres in 
diameter, and movement across the whole width of such a 
pupil could be readily watched at a distance of 4 to 6 
metres. But the diameter of the visual zone of the pupil, 



44 CONDITIONS OF ACCURACY. 

the only area in which the movement is of practical 
importance, is commonly much less than this, say from 4 
to 6 millimetres, and the movement of light across it can 
only be satisfactorily studied within the distance of two or 
three metres. 

Beyond one metre, however, the necessary inaccuracies 
of distance become usually of slight practical importance. 
In cases of aberration invading the central portions of the 
pupil and still more in cases of irregular astigmatism, the 
visual zone is considerably less in area than in the ordinary 
normal eye. In these cases, the test must be applied from 
a still shorter distance, often one-half or one-third of a 
metre, or even less. 

With the plane mirror it is easy to adopt any distance 
that suits the particular case. With the concave mirror 
any considerable variation in the distance requires a corre- 
sponding variation in the focus of the mirror used. A mir- 
ror of shorter focus being employed when the distance 
between the observer and patient must be short ; and of 
longer focus if a greater distance is to be maintained. 

The reason for this is that if the concave mirror be 
brought too close to the observed eye it gives an immediate 
source of light relatively too large, while if it be removed 
too far from the patient's eye, the diffusion is so rapid that 
it gives an illumination that is too feeble. These changes 
are much more rapid with the concave than with the 
plane mirror ; as one may readily demonstrate by holding 
both mirrors in his hand in the darkened room and reflect- 
ing areas of light, upon a wall from various distances. I 
have elsewhere (Journal of the Am. Med. Assoc, Sept. 4, 
1886) demonstrated the relations of the one to the other. 
In general the distance at which a concave mirror can be 



DISTANCE OF THE SURGEON FROM THE PATIENT 45 

used to best advantage is a little over four times its focal 
distance. 

For the majority of cases then, a distance of from ]/ 2 
to 2 metres is convenient for the plane mirror ; and one 
metre or a little less for the concave mirror, which should 
have a focal distance of from 20 to 25 centimetres. 

When it is desired to make the shadow test as accurate 
as possible, it is well to complete the test by placing before 
each eye lenses representing its supposed correction, with 
such addition to the convex or diminution of the concave 
spherical as shall bring the point of reversal to the great- 
est distance at which it can be satisfactorily studied in the 
particular eyes in question ; and then to test the movement 
of light and shade at that distance, looking especially for 
uncorrected astigmatism, and comparing the one eye with 
the other for any evidence of remaining inequality of 
refraction. 



CHAPTER IV. 

REGULAR ASTIGMATISM. 

The essential fact of regular astigmatism is that in two 
different directions, at right angles to each other [the prin- 
cipal meridians] , the curvature of the dioptric surfaces dif- 
fer, so that they exert unequal refractive power ; and that 
in all other directions, or meridians, the refractive power 
bears such a relation to the refractive power of these prin- 
cipal meridians, that it is only necessary to consider what 
happens in their direction. 

Two Points of Reversal. — In such an eye, the rays 
coming from the same point of the retina, and passing out 
through surfaces that refract unequally in different merid- 
ians, must leave the eye with different degrees of divergence 
or convergence in the directions of these different meridians. 
If the rays are convergent, or rendered so by passing through 
a convex spherical lens, they will be more convergent in 
one principal meridian than the other, and the point of 
reversal for one principal meridian will be at a different 
distance from the eye, from the point of reversal for the 
other principal meridian. The position of the point of 
reversal, giving the amount of myopia (either original or 
produced) in the principal meridian to which it belongs, 
the difference between the amounts of myopia in the two 
principal meridians will be the astigmatism. The general 
plan of measuring astigmatism by skiascopy, therefore, is 
to ascertain the point of reversal and measure the degree 

(46) 



THE BAND-LIKE APPEARANCE. 47 

of myopia for each principal meridian, and by subtracting 
the one from the other, to find the amount of regular 
astigmatism. 

The Band-like Appearance. — This difference in the 
position of the points of reversal for the different meridians, 
gives rise to certain phenomena of great practical import- 
ance in skiascopy. It is true of the astigmatic as of the 
non-astigmatic eye, that, as the point of reversal is ap- 
proached, the image of the retina seen through the pupil 
becomes magnified (see Chapter II). And, it necessarily 
follows that when the observer's eye is nearer to the point 
of reversal for one meridian than it is to the point of rever- 
sal for the other meridian, that the retinal image is more 
magnified in the direction of the principal meridian, to 
which the nearer point of reversal belongs. 

When the observer's eye is placed at the point of re- 
versal for one meridian, the retinal image becomes indefi- 
nitely magnified in the direction of that meridian, while 
comparatively little magnified in the direction at right 
angles to it. Each point of the retina then appears in the 
pupil as a line - running in the direction of that principal 
meridian , and the retinal light area, which consists of a 
number of these points, takes the form of an elongated band 
of light running in the direction of the principal meridian, 
which has its point of reversal at the observer's eye. This 
is the band-like appearance of the. light in the pupil, char- 
acteristic of astigmatism bounded by the " linear shadow " 
of Bowman. Figure 7 represents this appearance when 
the eye is placed at the point of reversal for one principal 
meridian, represented about twenty degrees from the 
vertical ; and figure 8 represents the appearance presented 
at the point of reversal for the other principal meridian, 
twenty degrees from the horizontal. Its direction is always 



48 



REGULAR ASTIGMATISM. 



that of the principal meridian, at whose point of reversal 
it is seen, and it is more pronounced, in proportion to the 
degree of astigmatism, the nearness of approach to the point 
of reversal, and the perfection of the focusing of the light 
upon the retina in the direction perpendicular to this prin- 
cipal meridian, that is, in the other principal meridian. 





Fig. 



Fig. 



In estimating astigmatism by skiascopy, two distinct 
things are to be done, which require different arrangements 
of the source of light. The first is to determine accurately 
the direction of the principal meridians by bringing out 
most distinctly this band-like appearance in the pupil, in- 
dicating the direction of one of these principal meridians ; 
the other being always, for regular astigmatism, at right 
angles thereto. The second thing to be done is to measure 
accurately the refraction in each of these principal merid- 
ians, testing them, of course, one at a time. 

The test proceeds at first as for myopia or hyperopia in 
a non-astigmatic eye, until a point of reversal is found. 
Then it is discovered that this point of reversal is only for 
the movement of light and shadow in one direction, and 
does not hold for movements at right angles to that direc- 
tion. The observer has now brought his eye to one point 
of reversal where the band-like appearance can be best per- 
ceived. But, as he has been working with the original 
source of light in the position most favorable for the meas- 



THE BAND-LIKE APPEARANCE. 49 

urement of hyperopia and myopia, the position that brings 
the immediate source of light as close as possible to the 
mirror (see Chapter III), he will probably see very little ap- 
pearance of the band in the pupil, even with the higher 
degrees of astigmatism. The reason for this is that with 
the immediate source of light in this position, the light is 
most accurately focused on the retina in the direction that 
the band should take. And, in the direction at right angles 
to the band, the focusing is quite incomplete, so that the 
diffusion at what should be the sides of the band partly or 
entirely neutralizes the effect produced by the magnification 
of the retina, which, otherwise, would cause the band-like 
appearance. 

In order to bring out this band-like appearance, it is 
necessary to make the focusing from side to side of the band 
as perfect as possible. And, to secure the perfect focusing 
in the principal meridians at right angles to the one in 
which the band is sought, the immediate source of light 
must be brought to the point of reversal for that other 
principal meridian. The band-like appearance is most per- 
fectly developed when the observer's eye is at the point of reversal 
for one 'principal meridian, and the immediate source of light at 
the point of reversal for the other principal meridian. 




Fig. 9. 

In figure 9, the solid lines represent the vertical merid- 
ian of an astigmatic eye and the rays emerging, so turned 
4 



50 REGULAR ASTIGMATISM. 

in that meridian, as to give the point of reversal at V. The 
broken lines represent the less curved horizontal meridian 
of the cornea, and the rays so turned in that meridian as to 
give a point of reversal at H. The dotted lines represent a 
plane mirror, P P, with the eye of the observer at V, and 
the light L pushed off from the mirror, so that the rays 
enter the eye as though they came from H, and are per- 
fectly focused on the retina in the horizontal meridian, ren- 
dering most distinct the appearance of a vertical band. 

For illustration, suppose a case (which the student will 
do well to reproduce for actual study, either in the artificial 
eye or by lenses placed before the living eye) having com- 
pound myopic astigmatism, the vertical meridian of the 
cornea being 2 D. myopic and the horizontal meridian 1 D. 
myopic. When, with the plane mirror, the observer's eye 
is one-half metre from the observed eye, it will be at the 
point of reversal for the vertical meridian, and in a position 
to see a vertical band of light. But, if the source of light 
be placed as close to the mirror as possible, the rays from 
it will be the more accurately focused upon the retina in 
the vertical meridian and more diffused horizontally, so 
that the real form of the retinal light area will be rather 
that of a horizontal line or band. 

Now, from the observer's position, the retina is most 
magnified in the vertical direction, and this vertical magni- 
fication would cause a point of light on the retina to appear 
as a vertical band in the pupil ; but, with the light area 
really in the form of a horizontal band, the effect of the 
magnification is largely neutralized and the appearance in 
the pupil may be quite indefinite. 

To bring out the band-like appearance : While keeping 
the observer's eye and mirror in the same position, the 
the original source of light must be pushed off from the mir- 



THE BAND-LIKE APPEARANCE. 51 

Tor one-half metre, the immediate source then retreats cor- 
respondingly behind the mirror, and approaches the posi- 
tion of the point of reversal for the horizontal meridian, 
one metre from the eye. 

With the light and mirror in this relation to the eye, 
the rays are focused upon the retina perfectly in the hori- 
zontal meridian and diffused in the vertical meridian, so 
that the real form of the retinal area of light is a vertical 
line or band. This vertical line or band being viewed from 
the point of reversal of the vertical meridian (where it will 
be greatly magnified in the vertical direction and but 
slightly magnified in the horizontal direction), gives rise to 
the appearance of the most distinct vertical band of light 
in the pupil. And, under these conditions, the presence of 
the astigmatism and the direction of one of its principal 
meridians is most clearly and accurately revealed. 

Taking the same case and using the concave mirror at 
a distance of one metre, which is the point of reversal for 
the horizontal meridian, the appearance of a horizontal 
hand of light in the pupil should be most distinctly visible. 
But, in order to develop it clearly, it will be needful to 
bring the original source of light so near to the mirror that 
the immediate source will be one-half metre in front of the 
mirror, that is, one-half metre in front of the observed eye, 
at the point of reversal for the vertical meridian. For it is 
from this position the light will be most perfectly focused 
•on the retina in the vertical meridian, while diffused in the 
horizontal meridian, and the horizontal magnification of 
the retina at the point of reversal for the horizontal merid- 
ian where the observer's eye is placed, will emphasize and 
increase the appearance of the horizontal band of light 
then thrown on the retina. 

Since, with the plane mirror, the immediate source of 



52 REGULAR ASTIGMATISM. 

light is always back of the mirror, and cannot be brought 
in front of it, the direction of the band can only be accu- 
rately determined for the meridian whose point of reversal 
is nearest the eye. It is only with the eye and mirror at 
this point of reversal that one is able, with the plane mir- 
ror, to bring the immediate source of light to the other 
point of reversal. And, with the concave mirror, since the 
immediate source of light is always in front of the mirror, 
the band-like appearance can only be distinctly brought out 
in the meridian which has its point of reversal the farther 
from the eye, as only with the eye at that point of reversal 
can the immediate source of light with the concave mirror 
be brought to the other point of reversal. 

With either the plane or concave mirror, only the band 
in one of the principal meridians can be most distinctly 
developed. But it is unnecessary in practice to bring out 
the bands in both meridians, since, by knowing the direc- 
tion of one principal meridian, the other being always per- 
pendicular to it, is also known. 

The measurement of the refraction in either of the 
principal meridians of astigmatism, is quite similar to the 
measurement of refraction in hyperopia and myopia. To 
determine whether the movement of light in the pupil in a 
certain meridian is with or against the movement of light 
upon the face, it is necessary that the focusing of the light 
on the retina be as perfect as possible in that particular 
meridian. To secure this, the immediate source of light 
must be as close as possible to the position of the observer's 
eye (see Chapter III). Hence, having determined the ex- 
istence of the astigmatism and the direction of its princi- 
pal meridians, the measurement in these meridians will 
proceed as the measurement of myopia or hyperopia. 



CHANGES IN THE LIGHT AREA. 53 

Changes in the Light Area at Different Distances. — 
In regular astigmatism, supposing the eye to be myopic in 
all meridians, or a convex lens placed before it sufficiently 
strong to over-correct the hyperopia in all meridians, the 
observer using a plane mirror and viewing the eye from 
different distances, will be able to recognize the following 
changes in the appearance and movement of the light in 
the pupil. 

From a position within the point of reversal of the 
more myopic meridian, the light will be seen to move with 
the light on the face, in all directions. As the observ- 
er's eye is withdrawn from the observed eye, and approaches 
the point of reversal for the more myopic meridian, the 
light area in the pupil becomes elongated in this meridian; 
and, while the movement is still with the light on the face 
in all meridians, it becomes more rapid in the direction of 
this elongation than in the direction perpendicular thereto. 

The observer, withdrawing his eye still farther on 
reaching the point of reversal for the more myopic merid- 
ian, [V, in figure 9,] is unable to distinguish the movement 
in this meridian, while the movement in the meridian at 
right angles to it is still with that of the light on the face. 

This point being reached, if the original source of light 
be pushed away from the mirror, so that its reflection, the 
immediate source of light approaches the point of reversal 
for the less myopic meridian, the form of the light in the 
pupil becomes a distinct band running in the direction of 
the more myopic meridian, readily seen to move from side 
to side, but without perceptible movement in the direction 
of its length. 

Bringing the source of light back to its usual position 
close to the mirror, and withdrawing his eye still farther 
from the eye under observation, the observer again sees the 



54 REGULAR ASTIGMATISM. 

movement of the light in the pupil in all directions. But 
in the direction of the most myopic meridian, it is now 
against the light on the face ; while in the meridian at right 
angles to this, it is still with the light on the face. The 
band-like appearance is now lost entirely ; the area of light 
in the pupil taking at one distance the same shape as 
though no regular astigmatism were present. 

But, as the point of reversal for the less myopic merid- 
ian is approached, elongation in the direction of that me- 
ridian may be noticed, and the erect movement of the light 
in that meridian becomes more rapid than the inverted 
movement now seen in the more myopic meridian. When 
the point of reversal for the less myopic meridian [H, figure 
9] is reached, the movement in its direction ceases, but it 
is impossible, at this point (with the plane mirror), to 
bring out so distinct a band as was seen in the direction of 
the other meridian. 

Withdrawing still farther, the light in the direction of 
the less myopic meridian begins to have an inverted move- 
ment, at first very rapid as compared with the movement 
in the more myopic meridian ; but, as the observer with- 
draws farther from this second point of reversal, the differ- 
ence in rate of movement in the two meridians becomes 
less noticeable. 

W T ith the concave mirror, the same series of appearan- 
ces are present, except that the directions of movement are 
reversed — " erect movement " meaning movement of the 
light in the pupil against the movement of the light on the 
face, and against the mirror; and u inverted movement " 
meaning the movement of the light in the pupil with the 
mirror and with the light on the face. With the concave 
mirror the meridian in which it is possible to bring out the 
band-like appearance of the light most distinctly is the 



MOVEMENT OF THE BANDS IN ASTIGMATISM. 55 

meridian of less myopia ; and it will be necessary to bring 
about the series of changes in the movement of the light 
area, which has been referred to, by changes of the lens 
placed before the eye, and not by changes in the observer's 
distance from the eye studied. 

Direction and Movement of the Bands in Astigma- 
tism. — The reason for the constant conformity of the di- 
rection of these bands of light to the principal meridians of 
refraction is obvious from their dependence on the magnifi- 
cation of the retina. That conformity sharply separates 
them from the somewhat similar appearance seen near the 
point of reversal in eyes free from astigmatism (page 32). 

O r ^o The apparent movement always 

L ^s. at right angles to their direction is 

/ dependent on an optical illusion, of 

/ which one may satisfy himself by 

.' making a hole in the centre of a 

/ Sheet of paper, holding behind this 

^ \ / hole the edge of a card, and moving 

it in a direction oblique to this edge. 

Fig. 10. qA| ie mo tion will appear to be in a 

direction nearly or quite perpendicnlar to the edge seen. 

Thus, in figure 10, the real movement of the card be- 
hind the opening, or the band of light behind the pupil, 
may be in the direction O o. But the movement will appear 
to be in the direction P p. 



w) 



CHAPTER V. 

ABERRATION AND IRREGULAR ASTIGMATISM. 

Iii astigmatism, strictly regular, though the refraction 
differs in different meridians, in any given direction or 
meridian it is the same at all parts of the pupil. In aber- 
ration and irregular astigmatism, the refraction differs in 
different parts of the pupil, even in the same meridian. 
All eyes present variations of this kind, and these varia- 
tions constitute an obstacle to the measurement of refrac- 
tion by skiascopy or by any other method. 

Appearances of Irregular Astigmatism. — To the be- 
ginner with skiascopy, they constitute the most serious 
obstacle he has to encounter. For one who has thoroughly 
mastered the principles of the test and become familiar 
with the various appearances of light and shade in the 
pupil, mistakes due to aberration or irregular astigmatism 
are readily avoided, while the reason for any uncertainty as 
to the results obtained by other methods, or any failure to 
secure perfect vision, on account of these defects is revealed. 

If we suppose two parts of the pupil, one of which has 
its point of reversal at the observer's eye, while the other 
is at a considerable distance therefrom, the illumination 
of the former will be the more feeble, of the latter the more 
brilliant ; the movement of the light in the former, if per- 
ceptible, will be rapid, in the latter, slow. If one watches 
two parts of the pupil, one of which has its point of rever- 
sal back of the observer's eye, and the other in front of it ; 

(50) 



IRREGULAR ASTIGMATISM. 57 

in the former the light will have a direct and in the 
latter an inverted movement. 

With the irregular astigmatism due to preceding cor- 
neal inflammation, or to the changes in the refraction of the 
lens that sometimes precede cortical cataract, the pupil ap- 
pears broken up into a considerable number of distinct 
areas, each of which has its separate movement of light and 
shadow, constituting the typical ophthalmoscopic or skia- 
scope picture of irregular astigmatism. The appearance 





Fig. ii. Fig. 12. 

caused by irregular astigmatism following corneal disease 
is shown in figure 11. That due to changes in the lens 
such as may precede cortical senile cataract is shown in 
figure 12, in which the black lines represent fixed spicules 
of actual opacity, while the other parts of the pupil indicate 
merely refractive differences, and change from light to dark 
or dark to light, as the inclination of the mirror is varied. 
Some such appearance is sometimes presented by young 
persons, indicating a congenital defect which may not 
noticeably increase in many years. 

If the differences of refraction in the different parts of 
the pupil are slight — that is, if the aberration or irregular 
astigmatism is of low degree — these differences of illumina- 
tion and movement will not be perceptible until the ob- 
server puts his eye close to the point of reversal. But at 
5 



58 ABERRATION AND IRREGULAR ASTIGMATISM. 

the point of reversal, they become perceptible and consti- 
tute a striking phenomenon in almost all eyes ; and, to the 
observer who does not understand their significance, one 
that is extremely confusing. In the nature and arrange- 
ment of its irregular astigmatism, every eye is peculiar. 
The number of varieties of play of light and shade that are 
obtainable as the point of reversal is reached, is equal to 
the number of eyes examined. Even the two eyes of the 
same individual differ. 

The only practical way to deal with irregular astig- 
matism by skiascopy is to understand thoroughly the gen- 
eral optical principles of the test, and apply them, so far as 
is needful, in the individual case. Certain peculiar forms 
of variations of the refraction of the eye in different parts 
of the pupil are, however, of sufficient constancy, regular- 
ity and practical importance, to warrant their separate 
classification and study. The most important of these is 
the regular, or symmetrical, aberration of the eye. 

Symmetrical Aberration. 1 — This is an error of the 
refraction of the eye which causes the rays of an incident 
pencil falling on the same meridian of the cornea, but at 
different distances from the axial ray, to meet at different 
distances behind the cornea ; while rays piercing different 
meridians of the cornea, at the same distance from the axial 
ray, intersect it at the same point. It is a defect similar to 
the aberration of convex and concave spherical lenses. It 
is readily recognizable in almost all eyes by skiascopy. In 
the majority of cases, it is in the same direction as ordinary 
spherical aberration ; that is, the margin of the pupil has a 
stronger lens action than the centre. The rays entering 
through the margin are brought to a focus first ; the rays 

1 For an account of this error of refraction see paper by the author, Trans. 
Amer. Ophthalmological Society, 1888, p. 141. 



SYMMETRICAL ABERRATION. 59 

entering nearer the centre being focused farther back. 
This is called positive aberration. 

In a certain proportion of cases, however, the defect is 
in the opposite direction ; the rays passing near the centre 
of the pupil being brought first to a focus, and those pass- 
ing through the periphery being focused farther back. The 
centre of the pupil has the stronger, and the periphery of 
the pupil the weaker, lens action. This is negative aberra- 
tion. 

The Visual Zone. — The variation of refraction, how- 
ever, does not usually proceed regularly from the centre of 
the pupil to the margin. But, as with spherical lenses, 
and to a greater degree, the central refraction is compara- 
tively uniform over a considerable area ; and, towards the 
margin the change of refraction becomes progressively more 
marked. This area in the centre of the pupil of compara- 
tively uniform refraction is the usual visual zone. It is the 
portion of the pupil that is of practical importance for pur- 
poses of distinct vision. Its size varies considerably. Some- 
times it includes almost the whole of the dilated pupil, in 
other eyes an extremely small area near the centre of the 
pupil will be regular, and the remainder of the pupil use- 
less for accurate vision. If a high degree of irregular 
astigmatism be present, the visual zone, instead of being a 
central area of considerable size, will often be some particu- 
lar portion of the undilated pupil, which happens to have 
the most regular curvature. 

In any case, for the correction of ametropia, it is the 
behavior of the light and shade in the visual area which 
has to be studied. Its behavior elsewhere may be disre- 
garded. It is often much easier to watch the movement of 
light and shade in some other portion of the pupil — some 
part of the extra-visual zone. And, if the observer does 



60 ABERRATION AND IRREGULAR ASTIGMATISM. 

not understand their relative importance, he will be apt to 
fix his attention on this latter and be led away from the 
true refraction of the eye he is examining. This is the 
more likely to happen, because in that part of the pupil, 
which has its point of reversal at, or near, the observer's 
eye, the direction of the movement of light and shade is 
difficult to see, while in other portions, the movement is 
more striking. 

The Appearances of Positive Aberration. — The ap- 
pearances presented by an ordinary case with positive aberra- 
tion may be considered in the order in which they will be 
developed with the plane mirror, the observer starting to 
examine the eye from within the point of reversal for the 
most myopic part of the pupil, and gradually withdrawing 
his eye until it is beyond the point of reversal for the least 
myopic part of the pupil. From the first position, the light 
area in the pupil is seen to move with the light on the face 
entirely across the pupil ; its motions in the edges of the 
pupil being more rapid and indefinite than in the centre. 
If, now, the observer's eye is withdrawn to the point of re- 
versal for the margin of the pupil, there appear in the mar- 
gin points in which no movement of the light can be seen. 
Some of these may be points of stationary light, and others, 
points of stationary shadow. 

As the observer's eye is still farther withdrawn, the 
points of stationary light run together and form a complete 
ring of light in the periphery of the pupil, shown in figure 
13, which is presently seen to have an inverted motion, to 
move against the light on the face. Within this is a ring 
of comparative shadow where the movement is swift and 
difficult or impossible to recognize ; and still within this 
lies an area of light, similar to that first seen, but now con- 
siderably reduced in size, which still moves with the light 
on the face. 



POSITIVE ABERRATION. 61 

As the observer draws still farther back, this area of 
light at the centre of the pupil, as shown in figure 14, 
grows smaller, and its movement more difficult to certainly 
distinguish. The ring of comparative shadow around it 
encroaches upon it, and the ring of light in the margin of 
the pupil in turn encroaches upon the shadow, and becomes 
brighter and its movement more readily noticeable, fig. 14. 





Fig. 13. Fig. 14. 

Withdrawing still farther, the point of reversal for the 
centre of the pupil is reached. The central area of light 
becomes faint and its movement ceases to be noticeable, 
the ring of feeble illumination surrounding it having swal- 
lowed it up. But, around this feeble light area, the ring 
of inverted movement has now grown broad and distinct. 
And, as the observer withdraws still farther, this ring of 
inverted movement closes in until it occupies the whole of 
the central area, and the observer sees an area of light 
moving across the whole pupil, having an inverted move- 
ment, that is against the mirror or light on the face. 

The movements of these erect and inverted light areas 
in the pupil are illustrated by figures 15 and 16. Figure 
15 shows the plane mirror turned to the left, or the concave 
mirror turned to the right, the central erect area being dis- 
placed toward the left, and the peripheral inverted area to- 
ward the right of the space it occupies. Figure 16 repre- 



62 ABERRATION AND IRREGULAR ASTIGMATISM. 

sents the light areas displaced in the opposite directions by 
an opposite inclination of the mirror. 





Fig. 15. Fig. 16. 

With the concave mirror, a similar series of changes 
may be brought about by placing before the eye successive 
strengths of the lenses, beginning with the weakest convex 
or strongest concave. The first should allow the points of 
reversal for all parts of the pupil to be back of the observer, 
and the successive changes bring these points closer and 
closer to the eye until all are in front of the observer. The 
movement is at first against the light on the face. Then 
appears the ring of illumination and swift movement in the 
margin of the pupil with the light on the face. The cen- 
tral area of light is then encroached upon by the ring of 
faint illumination, and this in turn by a ring of more bril- 
liant illumination in the margin moving with the light on 
the face, which latter finally occupies the whole area of the 
pupil. 

If the point of reversal be approached from the oppo- 
site direction, that is, starting with the observer's eye beyond 
it, we have, with the plane mirror, at first, inverted move- 
ment across the whole pupil. Then, as the point of re- 
versal for the centre of the pupil is approached, the light 
in the central zone becomes feeble and its movement indefi- 
nite. When the point of reversal for that part is passed,. 



POSITIVE ABERRATION. 63 

there appears, in this central zone, an erect movement of 
light and shade, at first rapid and hard to see, but growing 
slower, gaining in distinctness, and occupying a larger and 
larger part of the pupil as the patient's eye is approached, 
until, finally, it occupies the whole area. 

With the concave mirror, starting with the strongest 
convex or weakest concave lens, the movement is first with 
the mirror, throughout the pupil, then, as the lens is 
changed, it becomes indefinite at the centre ; presently it is 
against that of the mirror at the centre, while still with it 
at the margin ; and, with still weaker convex lenses, or 
stronger concaves, it becomes against throughout the whole 
pupillary area. 

Appearances of Negative Aberration. — With nega- 
tive aberration, the series of changes is apt to be less regu- 
lar and complete, and the picture presented by the pupil is 
less characteristic. But the succession of appearances is 
the reverse of what has been described for positive aberra- 
tion. 

With a plane mirror starting closer to the patient's eye 
than the point of reversal for the most myopic part of the 
pupil, the movement is with that of the light on the face 
throughout the whole pupil. As the observer's eye is with- 
drawn to a greater distance, this movement becomes indefi- 
nite, and the light feeble near the centre of the pupil. 
Presently, the movement at the centre of the pupil is lost, 
while still quite distinctly with that of the light on the face 
in the irregular ring-shaped area of the periphery. With- 
drawing still farther from the eye, the inverted movement 
at the centre of the pupil becomes distinctly visible, and 
the direct movement near the margin becomes more and 
more encroached upon and less and less distinct, until 
finally all erect movement is lost and we have only the in- 



64 ABERRATION AND IRREGULAR ASTIGMATISM. 

verted movement, which extends across the whole pupil. 
Before the erect movement entirely disappears, it is apt to 
break up into small areas detached from one another by 
spaces of comparative shadow, but still presenting some 
remnant of the erect movement. 

With the concave mirror, starting with a convex lens so 
weak, or a concave lens so strong, that the point of reversal 
is back of the observer, we have the direct movement 
against the light on the face throughout the pupil. The 
strengthening of the convex lenses or the weakening of the 
concaves, so as to bring the point of reversal closer to the 
observer's eye, causes : first, the fading of the light and the 
indefiniteness of its movement in the centre of the pupil, 
then the inverted movement, or with the light on the face, 
at the centre, and the area of this movement extending 
until it includes the whole of the pupil. 

Approaching the point of reversal from beyond it, we 
have, with the plane mirror, inverted movement through- 
out the whole pupil, giving place to indistinctness first at 
the margin. Then the indirect movement confined to the 
central area of the pupil and direct movement appearing at 
certain parts of the marginal area. This direct movement 
becomes more distinct and its area increases as the patient's 
eye is approached, until, at the point of reversal for the cen- 
tre of the pupil, all inverted movement is lost, and the erect 
movement is seen in all parts of the pupillary area. 

With the concave mirror, starting with the point of 
reversal between the observer and patient, and removing it 
successively farther from the patient, by the use of weaker 
convex or stronger concave lenses, we have first the move- 
ment with the light on the face throughout the pupil ; 
then indefiniteness at the pupillary margin, changing, in 
turn, to movement against the light on the face. The area 



NEGATIVE ABERRATION. 65 

of this peripheral movement then encroaches upon the 
central area until that is obliterated, and the movement 
against the light on the face occupies the whole width of 
the pupil. 

While the order of their development remains the 
same, the exact character of the appearances presented var- 
ies considerably with the degree of aberration. Generally, 
in the higher degrees, the areas of light occupy the greater 
part of the pupil and the area of feeble illumination separ- 
ating them is comparatively narrow. While in very low 
degrees of aberration, the area of feeble illumination is 
broad, and it may be difficult to recognize more than one 
of the light areas at one time. That is, when the area of 
erect movement is visible, the remainder of the pupil is 
occupied by the area of feeble illumination ; and when the 
area of inverted movement is developed, the area of feeble 
illumination so encroaches upon the area of direct move- 
ment that it can no longer be identified. 

In some eyes, the variation of refraction from point to 
point which constitutes symmetrical aberration, is almost 
or entirely confined to the periphery of the pupil. In these, 
the appearances characteristic of aberration are hard to de- 
velop. 

Appearance of Conical Cornea. — In other eyes, an 
opposite condition is present. The variations of refraction, 
instead of being confined to the periphery of the pupil, 
encroach upon the normal visual zone, confining it to a 
very narrow area. In these eyes, the skiascopic appear- 
ances of aberration are striking and characteristic, and one 
of them is that which has been regarded as peculiar to con- 
ical cornea. 

The error of refraction produced by conical, cornea is 
a high degree of negative aberration. At the apex of the 



G6 ABERRATION AND IRREGULAR ASTIGMATISM. 

cone, the curve is sharp, causing, usually, very high myopia 
in the corresponding part of the pupil. The sides of the 
cone, on the other hand, are comparatively flat, causing 
diminished myopia as the region of the apex is departed 
from and often running into hyperopia near the edge of the 
pupil. 

If the observer's eye be placed somewhere near the 
point of reversal for the periphery of the pupil, the move- 
ment of light in that portion of the pupil will be rapid, but 
the movement in the portion of the pupil corresponding to 
the apex of the cone will be slow. On account of the high 
myopia, the point of reversal for this part of the pupil is 
very close to the eye, and, generally, many dioptres removed 
from the observer's eye. The movement of light in the 
pupil, then, is slow near the centre and rapid towards the 
periphery, causing the area of light to appear to wheel 
around a fixed point corresponding to the apex of the cone. 
The light area is first seen on one side of the pupil, then on 
the other, but always rests upon the central fixed point. 

In certain positions of the light, the form of this area 
will be somewhat triangular, its base resting On the margin 
of the pupil and its apex at the apex of the corneal cone. 
Sometimes the triangle covers almost half of the pupil, in 
other conditions of light it is considerably narrower, but 
the constant and characteristic phenomena is the wheeling 
of the light area about the fixed point at the apex. 

This is shown in figures 17 and 18, which represent 
the appearance of the pupil with the mirror inclined in 
opposite directions. 

It was for the detection of these appearances, to which 
attention was called by Bowman, in 1857, that the test was 
first employed. Bowman mentions that he was able by 
means of it to detect low degrees of conical cornea, which 



APPEARANCE OF CONICAL CORNEA. 67 

would not be detected in any other way. It is certain that 
among those cases that have been classed as low degrees of 
conical cornea, on account of their presenting such appear- 





Fig. 17. F IG . 18. 

ances, a considerable proportion were not of conical cornea 
at all, but were cases of high aberration from other forms 
of defect in the dioptic surfaces. 

The appearances in question occur in all cases of high 
aberration. Where the aberration invades the central por- 
tion of the pupil, and is not confined to the periphery, the 
phenomena are quite as striking and characteristic, and of 
very much more frequent occurrence in cases of high posi- 
tive aberration than in cases of true conical cornea. The 
conditions for their recognition are that the observer's eye 
shall be comparatively near the point of reversal for the 
margin of the pupil, and comparatively far removed (esti- 
mating by dioptres) from the point of reversal for the cen- 
tre of the pupil. By careful management of the light and 
relative position of observer and patient, such appearances 
can be demonstrated in the majority of eyes. 

Like the band-like appearances of the light in astig- 
matism, those of conical cornea reveal the presence of the 
condition and the location of the apex of the cone', but be- 
yond this, they are of little value. The measurement of 
the difference of refraction between the margin and the 



68 ABERRATION AND IRREGULAR ASTIGMATISM. 

centre of the pupil, or the measurement of the refraction in 
the portion of the pupil best suited to purposes of vision, 
must be accomplished by the same application of skiascopy 
as serves to measure the amount of hyperopia or myopia in 
an eye free from astigmatism and aberration. 

The series of movements presented in positive aberra- 
tion can be well studied in one of the numerous forms of 
artificial eyes, in which spherical lenses are used to repre- 
sent the dioptric surfaces, and it is well by such study to 
become thoroughly familiar with them. They may, of 
course, be studied in living eyes presenting positive aberra- 
tion ; but, in many of these, the appearances are not so 
typical and regular in order of sequence, as with the ordi- 
nary strong spherical lenses. The appearances presented 
by negative aberration can only be studied in eyes in which 
this condition of the refraction is present, but their recog- 
nition and observation will be comparatively easy to one 
who has mastered the corresponding appearances of positive 
aberration, and who understands the optical conditions on 
which these appearances depend. 

Scissors-like Movement. — A special form of irregular 
astigmatism exists of sufficiently frequent occurrence and 
striking character to merit special description. It is also 
of some practical importance. In it, one portion of the 
pupil, as an upper or lower half, is more myopic in a cer- 
tain meridian than is the other part of the pupil. This 
causes an inverted movement of light in the one portion of 
the pupil, while there is an erect movement in the other. 
These two areas are distinct and separated by an intermedi- 
ate zone of feeble illumination. As the light is made to 
move back and forth in this meridian, the two areas of 
light in the pupil are seen alternately to approach and sep- 
arate, narrowing or widening the intermediate zone. As 



SCISSORS- LIKE MOVEMENT. 69 

the areas, under these circumstances, are generally band- 
like, or have comparatively straight margins, the effect 
is similar to that of the opening and closing of a pair of 
scissors. These appearances are represented in figure 19, 
which shows the mirror so turned as to separate the two 
areas ; and figure 20, which represents them brought to- 
gether by an opposite inclination of the mirror. Suppos- 
ing the upper part of the pupil to be more myopic, figure 
19 corresponds to the plane mirror facing down or the con- 
cave mirror facing up ; and figure 20 shows the plane mir- 
ror facing up or the concave mirror facing down. 





Fig. 19. Fig. 20. 

The relative size of the two areas will depend on the 
distance of the observer from the eye or upon the strength 
of the lens employed. As the observer withdraws to a 
greater distance, or the convex lens is made stronger, or the 
concave lens is made weaker, the area of inverted move- 
ment encroaches upon the zone of feeble illumination sep- 
arating the areas of light and the area of erect movement 
diminishes. As the observer comes closer to the eye, or 
the convex lens is made weaker or the concave lens stronger, 
the area of inverted movement diminishes. Always the 
observer's eye is at or near the point of reversal for the por- 
tion of the pupil occupied by the intermediate zone of 
feeble illumination ; and, in making the determination of 



70 ABERRATION AND IRREGULAR ASTIGMATISM. 

the refraction for practical purposes, care must be taken to 
see that this zone occupies a portion of the pupil that is 
available when the pupil is contracted, as under ordinary 
conditions of illumination and near work. 

The scissors-like movement may be produced in an 
artificial eye by placing the lens which represents the diop- 
tric surfaces, so that the light passes through it obliquely. 
It may also be developed in most eyes by applying skias- 
copy from some direction at a considerable angle to the 
optic axis. Its presence in the eye indicates obliquity of 
one or more of the dioptric surfaces. Probably it is often 
due to some obliquity in the position of the crystalline lens. 
Perhaps, because of such obliquity, this appearance of light 
and shadow in the pupil is apt to co-exist with a consider- 
able degree of regular astigmatism, which, on account of it, 
becomes more difficult to recognize and measure than it 
would otherwise be. Eyes presenting it, therefore, demand 
special care and attention on the part of the observer, to 
develop their best vision possible with correcting lenses. 



CHAPTER VI. 

PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

Position and Arrangement of Light. — The room being 
thoroughly darkened, the patient and surgeon take posi- 
tions facing each other at a distance of about one metre 
with the original source of light close to the surgeon on 
the side of the eye he desires to use, that is on the right if 
he intends using his right eye for the test. He can really 
use but one eye at a time, although he will find it much 
more pleasant to work with both eyes open if he once learns 
to do so. The source of light should be freely movable 
over a space of about one metre, a movement obtainable 
with a double jointed bracket of over one-half metre total 
length. The light is covered from the patient's face and 
also from the surgeon's except at the aperture of five [or 
ten] millimetres opposite the brightest part of the flame. 

The mirror is held so that with the eye he is using the 
surgeon can watch through the sight hole the movement 
of light on the patient's face and turned until the area of 
light that it reflects falls upon the eye to be tested. If dif- 
ficulty is experienced in properly directing the light, the 
surgeon may hold his hand a few inches in front of the 
mirror and upon it find the light area and get it properly 
directed towards the patient's eye. If the mirror be large 
it is necessary that the central portion of the light area be 
made to fall upon the patient's eye, the centre being marked 

(71) ' 



LL APPLICATION WITH THE PLANE MIRROR. 

by a spot of feeble illumination, corresponding to the sight, 
hole of the mirror. 

With the light properly directed, the pupil will be 
seen to be occupied by a red glare, the light area with 
which skiascopy is especially concerned. In first attempt- 
ing the test, care must be taken to discriminate clearly be- 
tween this general red glare and the reflection from the 
cornea or from the surfaces of any lens that may be placed 
before the eye. These reflections have the same color as 
the light used for the test. The one from the cornea is. 
small and brilliant, a mere point of light if the room be 
thoroughly darkened and the original source of light prop- 
erly shaded. The reflections from the lenses employed are 
larger and more confusing. They may be avoided by tilt- 
ing the lens slightly, in which case, they pass off to the peri- 
phery, leaving the centre of the lens free from reflection. 

Hyperopia. — If the mirror be rotated about a vertical 
axis, that is if it be made to turn more to the right or left, 
the area of light in the pupil will be seen to move with the 
light on the face to the right or left as the inclination of 
the mirror changes. If the rate of movement be slow, the 
hyperopia is of high degree, if more rapid, it is lower. 

A convex lens is now to be placed before the eye and 
this rate of movement of light in the pupil is the guide to 
the probable strength of lens required. If the observer has 
not sufficient practice with skiascopy to judge in this way 
about the strength of the lens required, he will save time 
by placing before the eye rather a strong lens, one of say 
5 D. With this the light is again thrown upon the eye, 
and if the lens be not sufficient to correct the hyperopia 
present, the movement of light in the pupil will still be 
found with that of the light on the face. In this case a 
still stronger lens must be used. This strengthening of 



HYPEROPIA. 73 

the convex lens before trie eye is continued until one is 
found which causes the reversal of the apparent movement 
of light in the pupil — until the light in the pupil moves 
against the light on the face. » 

Then the surgeon is to approach the patient, mean- 
while rotating the mirror and watching for the nearest point 
at which he still sees the inverted movement in the visual 
zone. Near this distance, the illumination of the pupil be- 
comes quite feeble, and the movement being rapid requires 
the closest watching. Approaching still nearer to the 
patient the light in the visual zone is seen to move with the 
light on the face, and the greatest distance at which this 
can be distinguished is to be noted. Between these two, 
the least distance of inverted movement, and the greatest 
distance of direct movement lies the point of reversal. 
But, for reasons given page 40, it is better to take the lat- 
ter, the greatest distance at which direct movement can be 
perceived as the point sought. 

The distance from the surgeon's to the patient's eye 
is then measured. It is the focal distance of the amount of 
myopia produced by the convex lens employed. That 
amount is to be subtracted from the total strength of the 
lens to ascertain the portion of its strength which has 
been necessary to correct the existing hyperopia. 

Having made such a determination of the refraction 
and having repeated the various observations until no 
doubt is left as to their correctness, the lens before the eye 
is to be changed for one sufficiently weaker to carry the 
point of reversal to as great distance as the size of the 
visual zone will allow the accurate determination of the 
movements of light and shade within it. At this distance 
the final estimate of the ametropia is to be completed. 

For example, suppose the eye under examination to 
6 



/ 4 APPLICATION WITH THE PLANE MIRROR. 

have hyperopia of 3 D. When the 5 D. lens is placed be- 
fore it, the point of reversal will be brought to one-half 
metre. As the surgeon's eye is made to approach that of 
the patient, the inverted movement in the visual zone will 
cease when they are about 60 centimetres apart. Going 
still closer, the erect movement will be distinguished at 
about 40 centimeters. These observations are to be repeated 
until the surgeon makes sure that the point of reversal lies 
somewhere between 40 and 60 centimetres. The 5 D. lens 
is then replaced by the 4 D. lens. Repeating the test, the 
inverted movement is seen at one and one-quarter metres 
and the direct movement as far away as one metre, thus 
locating the point of reversal at about 1 metre from the 
eye, and determining the myopia caused by a 4 D. convex 
lens to be 1 D. and the refraction of the eye to be 4 D.- 
1 D.=3 D. of hyperopia with less than 0.25 D. of possible 
error either way. 

Myopia. — In myopia the first rotation of the mirror 
will usually cause a movement of light in the pupil against 
that of the light on the face. The surgeon then approaches 
the patient, continuing the movements of the mirror and 
w r atching the apparent movement of light in the pupil, 
until this apparent movement becomes rapid and indefinite 
and presently is entirely lost. Approaching still closer to 
the patient's eye, the movement of the light area in the 
pupil again becomes distinct, but is now with the move- 
ment of the light on the face. Drawing back again, the 
surgeon notes the greatest distance at which this erect 
movement can be observed, and the shortest distance at 
which the inverted movement is distinguishable, and takes 
a point midway between these to be the point of reversal. 

The distance of this point of reversal from the patient's 
eye is the focal distance of the lens that will be required to 



MYOPIA. 75 

correct trie myopia. To complete the test, however, a lens 
about i D. weaker than this is placed before the eye to 
bring the point of reversal to the distance of a metre and 
the test is repeated, the surgeon noting carefully the great- 
est distance at which the erect movement is visible, and 
the shortest distance at which the inverted movement is 
perceived, always in the visual zone. The distance of the 
point of reversal as thus determined is the focal distance of 
the lens required to correct the remaining myopia. The 
strength of such a lens added to the strength of the lens 
already before the eye, gives the total amount of myopia 
present. 

Suppose the eye to be 6.5 D. myopic. With the first 
test the inverted movement will be perceived up to about 
eight inches from the. patient's eye ; and at five or six inches 
from the eye an erect movement will begin. From this, 
the surgeon assumes that the myopia is about 7 D. [focal 
distance 6 1 /, inches] and he will, therefore, place before the 
eye, for the more accurate test, a concave 6 D. lens. On 
trying the movement of light in the pupil through this 
lens, it will be found at the distance of one metre to be 
with that of the light on the face. The surgeon then with- 
draws still farther from the patient until the direct move- 
ment becomes indistinguishable and at two metres is entirely 
lost. Drawing back still farther from the patient, he will 
in a favorable eye be able to distinguish the inverted move- 
ment in the pupil, and in this way fix the point of reversal 
at a distance of two metres, indicating with great accuracy 
an uncorrected myopia of 0.5 D. 

Sometimes, however, the distance of two metres will 
be found so great that it is difficult or impossible there to be 
sure of the movement in the visual zone. In such a case 
the 6 D. lens will need to be replaced by a weaker lens as 



76 APPLICATION WITH THE PLANE MIRROR. 

a 5.5 D., with which the erect movement will be seen to 
almost a metre, and the inverted movement again a few 
inches beyond that point. 

If the myopia be very low, the first inspection of the 
pupil without a lens may show a movement of light in it 
with the light on the face. In such a case, the surgeon 
will draw back as far as he can readily distinguish the 
movement of light in the visual zone. If the movement 
still appears to be with that of the light on the face, he will 
place before the eye a convex lens, and with it determine the 
point of reversal as for a case of hyperopia. The final 
result of testing, however, will show that the myopia caused 
by the lens is greater than the strength of the lens, and, 
therefore, that some myopia must have been present before 
the lens was placed in front of the eye. 

For example, suppose that before reaching that dis- 
tance of two metres the erect movement in the pupil 
becomes indistinct, but that the visual zone, where the 
movement must be watched, is so small that beyond this 
the direction of movement in it cannot be recognized with 
certainty. A 0.5 D. convex lens being placed before the 
eye is found to cause an inverted movement up to 125 cen- 
timetres, and to confine the erect movement to within 85 
or 90 centimetres of the eye. The point of reversal then, 
is at one metre. The amount of myopia corresponding to 
this is 1 D., of which 0.5 D. w r as the amount originally 
present in the eye. 

Emmetropia. — On first inspection, without a lens, the 
surgeon sees an erect movement in the pupil, the rapidity 
of which indicates that if there be hyperopia it is of low 
degree. Drawing back from the patient's eye as far as pos- 
sible, however, this erect movement still continues. He 
places before the eye under observation a convex lens of 1 



EMMETROPIA. 77 

or 2 D., and viewing the movement of light in the pupil 
through this lens, finds where the inverted and the erect 
movement come together. On measuring the distance of 
this point of reversal from the patient's eye, he finds that 
it exactly corresponds with the focal distance of the lens he 
has been using. That is, the lens has caused myopia just 
equivalent to its own strength, showing that before they 
passed through the lens, the rays emerging from the cornea 
were parallel. 

Regular Astigmatism. — Whether it be known that 
the eye under examination is astigmatic or not, the test 
will proceed at first as for simple hyperopia or myopia. 
Sometimes if the astigmatism be high and one meridian 
nearly emmetropic or slightly myopic, the first inspection, 
without any lens, will reveal an unmistakable band of light, 
or that there is erect movement in one meridian and in- 
verted movement in another, or that the movement of 
light in the pupil is more rapid in some one meridian than 
in the meridian at right angles to it, indicating that these 
meridians have different points of reversal, and that the 
surgeon is nearer the point of reversal for the former than 
for the latter. 

But, commonly, the first appearance will give no posi- 
tive indication of the presence of astigmatism, and the test 
goes on until a point of reversal is found. Then, on trying 
the movement of light and shade in different meridians, as 
should always be done from the neighborhood of the point 
of reversal, it is discovered that it is the point of reversal 
for only one meridian ; and that for the meridian at right 
angles to that one, there is a distinct movement of the light 
either erect or inverted. 

If the movement still noticeable from the point of 
reversal first discovered be an inverted movement — against 



78 APPLICATION WITH THE PLANE MIRROR. 

the light on the face — the surgeon should bring his eye 
still closer to the patient until this inverted movement 
ceases. He will then be near the point of reversal for the 
meridian in which the inverted movement was before 
noticed, and will be able to see' in the other meridian an 
erect movement. 

Such a lens is now to be chosen and placed before the 
eye as will bring this point of reversal for the more myopic 
meridian — the point of reversal from which an erect move- 
ment is seen in the other meridian — to a convenient distance 
from the eye. The surgeon's eye is placed as nearly as pos- 
sible at this point of reversal. Then the original source 
of light [which has up to this stage of the test accompanied 
the mirror in its movements to or from the patient's eye] 
is pushed away from the mirror, and while it is pushed 
away, the mirror is rotated and the light area in the pupil 
watched. This light area will be seen to assume the band- 
like appearance characteristic of astigmatism. 

At a certain distance this band-like appearance will be 
most distinct. With the source of light nearer the mirror 
or farther from the mirror, it will be less characteristic. 
The distance of the light from the mirror at which the 
band becomes most distinct is the distance between the two 
points of reversal. The surgeon's eye (with the mirror) is 
now at the point of reversal for the more myopic meridian, 
and the immediate source of light is at the point of rever- 
sal for the less myopic meridian. 

With the light in this position, the direction of the 
band is to be carefully studied and noted as the direction 
of one of the principal meridians of astigmatism. It is the 
direction of the axis of the convex cylinder that will cor- 
rect the astigmatism. The other principal meridian will* 
of course, be perpendicular to this. 



REGULAR ASTIGMATISM. 79 

Having now fixed the direction of the principal me- 
ridians of astigmatism, the surgeon should again bring the 
original source of light as near to the mirror as possible, 
and proceed to measure the refraction, first in the one prin- 
cipal meridian and then in the other, just as he would 
measure the refraction in cases of hyperopia or myopia. 
The difference between the refractions of the two being the 
amount of astigmatism present. 

To measure the refraction in a certain meridian the 
light is made to move on the face and on the retina in the 
direction of this meridian by rotating the mirror about an 
axis perpendicular to it. Thus for the vertical meridian 
the light is made to move vertically by turning the mirror 
about a horizontal axis. For the horizontal meridian the 
light is made to move horizontally about a vertical axis. 
Great care is necessary in the higher degrees of astigmatism 
to make the movement conform accurately to the meridian 
to be tested, since any oblique movement will appear (see 
page 55) as though perpendicular to the band. 

When the astigmatism is of very low degree, 0.5 D. or 
less, it becomes correspondingly difficult to distinguish be- 
tween the points of reversal for its principal meridians. 
The band-like appearance of the light in the pupil becomes 
less characteristic, and there is no space between the two 
points of reversal where an erect movement can be obtained 
in the direction of one meridian, and a reverse movement 
in the direction of the meridian perpendicular to it. In 
these cases, the astigmatism is to be recognized by the fact 
that when near one point of reversal, the movement in one 
meridian has become indistinguishable, it can still be per- 
ceived in the other principal meridian. And, if the sur- 
geon places his eye at the point of reversal for the more 
myopic meridian and pushes the source of light a little 



80 APPLICATION WITH THE PLANE MIRROR. 

away from the mirror, the erect movement in the meridian 
of less myopia, and absence of movement in the more 
myopic meridian becomes most distinct. It is upon the 
behavior of light in the pupil under these conditions that 
the diagnosis of the very low degrees of astigmatism must 
principally rest. 

The final test in any case will be made with the points 
of reversal brought together, usually at a distance of i 
metre or more.* To do this, it will be necessary to place 
before the eye such a cylindrical lens as will correct the 
astigmatism, together with the spherical lens which will 
bring the point of reversal to the desired distance. With 
these lenses before the eye, the test is again applied. If 
the light in the pupil is found to move with the light on 
the face, the surgeon withdraws to a greater distance until 
that movement becomes indistinct. If the movement in 
the pupil is found against that of the light on the face, the 
surgeon approaches the patient until the movement be- 
comes indistinct. The apparent movement is to be care- 
fully inspected from the point of reversal and from a little 
within and a little beyond it. 

If it is found that the reversal occurs at the same dis- 
tance from the eye for all meridians, the cylinder chosen is 
known to be correct, both as to strength and as to the 
placing of its axis ; and the distance of this point of rever- 
sal from the eye indicates the amount of myopia which the 
spherical lens employed has caused or has left uncorrected. 

If, however, the movement of light is found to cease 
in some meridian, but to continue (either direct or inverted) 
in a meridian at right angles thereto, it becomes evident 
that the cylinder chosen does not perfectly correct the 
astigmatism. If the astigmatism thus found to remain has 
the same principal meridians as those already fixed upon, 



REGULAR ASTIGMATISM. 81 

"the direction of the axis of the lens is correct, but its 
strength is not exactly right. Whether the strength needs 
to be increased or to be diminished will appear from the 
fact that the more myopic meridian continues to be the 
more myopic ; or that what was originally the less myopic 
meridian has become the more myopic. 

If the astigmatism remaining after the cylindrical lens 
has been placed before the eye has principal meridians that 
do not correspond with those for which the lens is placed, 
the placing of the lens is incorrect, and the direction of its 
axis needs to be slightly varied, until the remaining astig- 
matism disappears or its direction corresponds with that of 
the lens before the eye. 

Where the cylindrical lens before the eye is of the 
right strength or is too weak, its axis needs to be turned 
slightly toward the axis of a similar cylinder which would 
correct the remaining astigmatism. If the cylindrical lens 
already before the eye is too strong, its axis needs to be 
turned slightly toward the axis of a cylindrical lens of the 
opposite kind that would correct the astigmatism. 

The effect of such combinations of cylindrical lenses 
may be more fully understood by the study of the writer's 
paper upon " The Equivalence of Cylindrical and Sphero- 
cylindrical Lenses " in the Transactions of the American OpJi- 
thalmological Society for 1886, page 268, or " Some Remarks 
on the Refractive Value of two Cylinders " by Carl Weiland 
Archives of Ophthalmology, 1893, p. 435, and 1894, page 28. 

W T hen the meridians of an}' remaining astigmatism 
liave thus been made to conform to the direction of the 
-cylindrical lens before the eye, this remaining astigmatism 
has to be corrected by a change in the strength of the cylin- 
drical lens. 

For example : suppose an eye having a compound 



82 APPLICATION WITH THE PLANE MIRROR. 

hyperopic astigmatism corrected by + i sph. 3 + J C )'I 
axis 95 . The first inspection of the movement of light 
in the pupil shows a movement with that of the light on 
the face in all meridians ; and the difference in the rate of 
movement in the different meridians will be so slight as 
probably to escape notice. A convex 3 D. spherical lens 
will cause the movement in the pupil to be against the light 
on the face in all meridians when the eye is viewed from a 
greater distance than one metre. But it will also be noticed 
that the light moves more swiftly from side to side than it 
does upward and downward. 

If now the surgeon brings his eye closer to the patient, 
when the distance of one metre is reached, the movement 
of the light from side to side becomes indistinguishable, 
while there is still a very distinct movement against the 
light on the face upward and downward. Approaching 
still closer, the movement from side to side is seen to be 
with, the movement of the light on the face, the inverted 
movement still continuing in the vertical meridian. The 
movement horizontally with the light on the face, at first 
very rapid, grows slower as the patient's eye is approached, 
and the movement — against the light on the face — in the 
vertical meridian grows more rapid, until at a distance of 
one-half of a metre, the movement in the vertical meridian 
becomes indistinguishable, although there is a very clear 
movement of light with the light on the face from side to 
side. 

The point of reversal for the more myopic meridian 
(more myopic with the lens) has now been reached, and the 
the surgeon keeping his eye at this position pushes the 
source of light away from the mirror. As he does so, the 
area of light in the pupil assumes more and more the 
appearance of a distinct vertical band, readily moved from 



REGULAR ASTIGMATISM. 83 

side to side, but without apparent movement in the direc- 
tion of its length. This band continues to become more 
and more distinct until the original source of light is one- 
half metre from the mirror ; and the immediate source 
consequently one-half metre back of the mirror, and one 
metre from the patient's eye — at the point of reversal for 
the less myopic meridian. In this position careful obser- 
vations will show that the band of light in the pupil is not 
exactly vertical, but has the direction corresponding to the 
more myopic meridian of 95 . The principal meridians 
then are located at 5 and at 95 °. 

Having determined this, the light is brought back as 
close to the mirror as possible, and the point of reversal for 
the 95 meridian is determined. To do this it may be 
advisable to change the convex spherical lens before the 
eye, but whatever lens is employed, from the results 
obtained with it, the surgeon deduces the fact that in that 
meridian the refraction of the eye is hyperopic 1 D. He 
then proceeds to measure in the same manner the refraction 
of the eye in the other principal meridian, finding with the 
convex 3 D. lens that this point of reversal is at one metre, 
and its refraction, therefore, hyperopic 2 D. The differ- 
ence between these meridians will be 1 D., the amount of 
astigmatism present. 

To make the final determination there, there should be 
placed before the patient's eye the 1 D. convex cylinder 
with its axis at 95 and a 2 D. convex spherical lens, with 
which the point of reversal for all meridians will be found 
to lie one metre from the eye. If, in the placing of the 
cylinder, its axis is made to not correspond exactly with 
the meridian of least hyperopia, there will be found by 
this test a remaining astigmatism of low degree. Suppose 
through carelessness or inaccuracy in the earlier observa- 



84 APPLICATION WITH THE PLANE MIRROR. 

tion, the axis of the cylinder should be placed at 105 in- 
stead of 95 , the remaining astigmatism then would be 
found to be such as would be corrected by a convex cylin- 
der with its axis at about 70 . But on turning the cylin- 
der before the eye io° in that direction, that is, to its proper 
direction at 95 , the remaining astigmatism would disap- 
pear. If, however, instead of the 1 D. cylindrical lens, a 
lens of 1.5 D. had been placed with its axis at 105 °, there 
would remain an astigmatism which might be corrected by 
a concave cylinder with its axis at about 70 , and the turn- 
ing of the cylinder before the eye io° in that direction 
[to 95 ] would cause the remaining astigmatism to so 
change that its meridians would be at 5 ° and 95 °, where a 
measurement of it would reveal the fact that the cylindri- 
cal lens employed was 0.50 D. too strong. 

Aberration and Irregular Astigmatism. — The differ- 
ence in the refraction of different parts of the pupil is to be 
ascertained by measuring the refraction for each part sep- 
arately, just as though it were a case of simple hyperopia 
or myopia, care being taken to confine each observation 
strictly to the little portion of the pupil the refraction of 
which it is desired to ascertain. 

The amount of aberration or irregular astigmatism 
that is thus ascertained is of some scientific interest and 
occasionally of practical importance as bearing on the 
prognosis of conical cornea, or of the changes of refraction 
in the lens which precede cataract. 

Generally, however, the important practical point 
about aberration or irregular astigmatism is its distribution. 
For practical purposes, the surgeon desires to ascertain 
which part of the pupil is free from any such defect, as 
that part will furnish the best visual zone ; and by what 
lenses that visual zone can be made most useful to the 



ABERRATION AND IRREGULAR ASTIGMATISM. 85 

patient. The need for careful study to develop these points 
is sometimes great. Figures 21 and 22 represent the 
appearances brought out by thorough investigation of a case 
of considerable astigmatism, coincident with equally pro- 
nounced positive aberration. Without careful study of the 
visual zone at the proper distances, it would have been easy 
to set the case down as one of aberration, and to have over- 
looked the astigmatism entirely. If the aberration en- 





Fig. 21. Fig. 22. 

croaches decidedly upon the area of the pupil as deter- 
mined by a moderate light, it may be necessary to give a 
correcting lens, for use at near work and on exposure to 
bright light, different from the one required when the pupil 
will be somewhat larger. Or the surgeon may need to 
caution the patient that under certain conditions of light 
he must expect the correcting glasses to give slightly im- 
perfect vision. 

A fact to be borne constantly in mind in the applica- 
tion of skiascopy is that it is not the high degrees of aber- 
ration and irregular astigmatism that are of most practical 
importance, requiring the surgeon to take into account 
their bad effects. More frequently it is the slight imper- 
fections of this kind situated within the portion of the 
pupil used for accurate vision that need to be recognized and 
taken into account, when prescribing glasses or in giving 



86 APPLICATION WITH THE PLANE MIRROR. 

an opinion as to the value of glasses. These low degrees 
of imperfection are to be recognized and studied only when 
the surgeon's eye is close to the point of reversal for the 
visual zone after the effects of hyperopia, myopia and regu- 
lar astigmatism have been excluded by placing the proper 
glasses before the patient's eye. 

The investigation of aberration and irregular astigma- 
tism is the last step in skiascopy. A step very frequently 
not taken, yet essential to complete certainty and accuracy 
in the objective measurement of refraction. In a small pro- 
portion of cases, it will lead to modification of the glasses 
previously selected as best, and in a much larger proportion 
of cases it will discriminate sharply between the lenses 
which really best correct the ametropia and others which 
appear to give equally good, or almost equally good sub- 
jective results. 

To carry it to completion will often require more time 
and effort than has been necessary for all other parts of the 
skiascopic examination. Nor is this strange, for it often 
includes the measurement of hyperopia or myopia with 
astigmatism in two or more areas of distinctly, though 
slightly, different refraction. It is not a distinct application 
of the test but its application to parts of the pupil, instead 
of to the pupil as a whole. It requires no special directions 
and cannot be much elucidated by examples. It is to be 
mastered by a full understanding of the optical principles 
of the test, Chapter II, strict observance of the conditions 
of accuracy set forth in Chapter III, and the exercise of the 
needful care and patience. 

Measurement of Accommodation. — The objective 
determination of the nearest point for which the eye can 
be focused is possible only by skiascopy. It is sometimes 
of importance as in cases of suspected cycloplegia in child- 



MEASUREMENT OF ACCOMMODATION. 87 

Ten, or others for whom the subjective test cannot be 
relied on. In determining the condition of the accommo- 
dation in an eye with imperfect vision, or in recognizing 
any slight remaining accommodation after the use of a 
mydriatic, the test is also of practical value. 

To make it, the surgeon first ascertains the refraction 
of the eye, and then places before it such lens or lenses as 
will correct astigmatism and bring the point of reversal to 
a distance of one metre or a little less. He then places 
himself at this distance from the patient, and directs the 
patient to fix his gaze upon some object on the farther side 
of the room in such a position that the visual axis of the 
eye under examination shall pass as close as possible to the 
surgeon's eye. The point of a finger or pencil is then held 
close to the patient's eye, about the near limit of conver- 
gence and in the visual axis, so that the direction of the 
visual axis shall not be changed during the test. 

The patient is then directed to look first at the object 
across the room, then at the point of the pencil close to his 
eye. The surgeon by watching his other eye can ascertain 
whether the movements of convergence are really executed. 
Very strong convergence being impossible without strong 
accommodative effort, if any power of accommodation 
remains the eye will be seen to grow more myopic when 
the pencil is looked at, and less so when the distant object 
is fixed, an inverted movement of the light in the pupil 
becoming apparent in the one position and disappearing in 
the other. 

To measure the amount of accommodation the sur- 
geon may approach the observed eye until the point of 
reversal is reached for the eye during very strong conver- 
gence, or the lens before the eye may be modified in the 
direction of weaker convex or stronger concave until the 



88 APPLICATION WITH THE PLANE MIRROR. 

point of reversal is brought with a new lens, with the 
accommodation, to the same distance as it was brought by 
the original lens without accommodation. The difference 
between the lenses in this case representing the amount of" 
accommodation present. 

For example : in a case of hyperopia 2 D. to ascertain 
if accommodation was present a convex 3 D. lens would be 
placed before the eye. This would bring to one metre the 
point of reversal of the eye with its accommodation relaxed. 
The surgeon at a little less than a metre could get through 
it an erect movement of the light in the visual zone when 
the patient was looking across the room. If now, on look- 
ing at the point of a finger held two inches in front of the 
eye, the movement becomes distinctly inverted, the light 
in the pupil moving against the light on the face, it is 
known that accommodation is present. In place of the 
3 D. lens a weaker convex may be substituted, and if the 
strong convergence of the visual axis still brings an in- 
verted movement of light in the pupil, a still weaker lens 
for this. In this way if it be found that with the 1 D. lens 
the patient is able by strong convergence and coincident 
accommodative effort, to bring the point of reversal to just 
one metre, the difference between the 3 D. lens and the 
1 D.=2 D. will be the amount of accommodation pres- 
ent. 

Instead of changing the lens, the surgeon can approx- 
imately estimate the amount of accommodation by bring- 
ing his eye closer to that of the patient and finding the 
new position of the point of reversal caused by the exer- 
tion of the accommodation. Where much accommodation 
is present, such an approximate determination should first 
be made, but it is open to the inaccuracies attendant on 
any skiascopic determination at a short distance. 



CHAPTER VII. 

PRACTICAL APPLICATION WITH THE CONCAVE MIRROR. 

The Source of Light. — The room is to be thoroughly 
darkened, and to secure this, it is well to have the original 
source of light shaded. This light will, however, usually 
be back of the patient and except for the determination of 
the principal meridians of astigmatism, the farther it is 
behind the patient the better, so that it is less essential to 
have the light thoroughly shaded. It is also of less import- 
ance that the original source of light should be small ; 
still the separation of light and shade should be as sharp 
as possible, so that the opaque shade with an opening oppo- 
site the brightest part of the flame will be found service- 
able. The opening in the shade may be two or three cen- 
timetres in diameter, so long as the original source of light 
is a metre or more away from the mirror. But when this 
is brought near the mirror to bring out the band of light 
in astigmatism, the opening should be one centimetre in 
diameter or less. 

The surgeon places himself a little over one metre 
from the patient. On throwing the light upon the patient's 
face with the mirror, it is found that the area of light 
on the face moves with the mirror just as in the case 
of the plane mirror. The same reflections from the cornea 
and trial lenses are to be recognized and provided for, and 
within the pupil lies a similar portion of red fundus reflex 
bounded by shadow, which is the subject of observation 
7 (89) 



90 APPLICATION WITH THE CONCAVE MIRROR. 

during the test. If, however, the light in the pupil be seen 
to move with the light on the face, the eye is myopic more 
than i D. If the light in the pupil be seen to move against 
the light on the face, the eye is hyperopic, emmetropic, or 
myopic less than I D. 

Hyperopia. — If the mirror be rotated about a vertical 
axis from right to left, the area of the light in the pupil 
will be seen to move from left to right, that is, against the 
mirror and against the light on the face. That this is really 
an erect movement, we know from the demonstrations as 
to the real direction of the movement of the light on the 
retina in Chapter II. The difference between erect move- 
ment and movement with the light on the face must be borne 
in mind. With the concave mirror, the one is just the 
opposite of the other. The movement with the light on 
the face being an inverted movement, and the movement in 
the pupil against the light on the face the erect movement. 

As with the plane mirror, the movement will be swift if 
the hyperopia be of low degree, slower if of higher amount. 
The convex lens is now to be placed before the eye, the 
swiftness of the movement in the pupil being the guide to 
the strength probably required. If the observer is not able 
to judge by this movement, let him at first employ in suc- 
cession lenses that differ considerably in strength, as the 2, 
4 and 6 D., increasing the strength as long as the move- 
ment in the pupil is against the movement on the face. 

When a lens is reached that causes movement of light 
in the pupil with the light on the face, slightly weaker 
lenses are to be tried until two consecutive lenses are found, 
of which one gives the movement against the light on the 
face and the next stronger causes movement with the light 
on the face. Between these two lies the lens strength 
which would bring the point of reversal to the surgeon's 



HYPEROPIA. 91 

eye. This being one metre from the patient, it is the lens 
which causes i D. of myopia ; and by subtracting i D. 
from its strength the hyperopia of the eye is obtained. 

For example : suppose hyperopia of 4 D. The light 
in the pupil will move against the light on the face at the 
first inspection, and also with the convex 2 D. and 4 D. 
lenses. With the convex 6 D. it is found to move with the 
light on the face. ' On trying the convex 5 D. the move- 
ment is indeterminable. With the 4.75 D. it is very rapid, 
but still against the light on the face. With the 5.25 D. 
it is equally rapid but with the light on the face. The lens 
strength between the two, or the 5 D., is then the one which 
causes 1 D. of myopia, and 5 D. the strength of the lens 
minus 1 D. the myopia caused by it, leaves 4 D. the lens 
strength required to correct the hyperopia present. 

Myopia. — In the mass of cases, inspection without 
a lens will show movement of light in the pupil with the 
movement of the light on the face, indicating that the 
point of reversal is between the surgeon and the patient. 
When this is the case, concave lenses are to be tried, their 
strength being indicated by the rate of movement, or if 
this be not a sufficient guide, they may be tried in series 
with an interval of 2 D., or more, until one is found which 
causes the light in the pupil to move against the light on 
the face. 

As the point of reversal is thus brought nearer to the 
observer's eye, the light area in the pupil becomes more 
brilliant and its movement more rapid. When the lens has 
been found which causes the light in the pupil to move 
against the light on the face, slightly weaker lenses are to 
be tried until it has been certainly ascertained which is the 
weakest lens that will cause the movement against that of 
the light on the face, and which is the strongest lens that 



92 APPLICATION WITH THE CONCAVE MIRROR. 

still allows movement in the pupil with the light on the 
face. Between these two lies the lens strength which leaves 
the eye i D. myopic ; this lens strength added to i D. will 
give the total myopia present. 

For example : suppose the myopia present to be 8.5 D. 
The movement in the pupil without any lens will be very 
slow, and the light area round and dim. Judging from this 
appearance, the first lens tried may be the concave 5 D. 
With it the light in the pupil will appear more brilliant 
and its movement will be more rapid, but it will still be 
with the movement of the light on the face. Next, the 
concave 8 D. will be tried. The movement of light will 
be found still more rapid, but now against that of the light 
on the face. With the concave 7 D. it will be found 
equally rapid, but with the light on the face. With the 
7.5 D. it will not be distinguishable. Hence the 7.5 D. lens 
leaves 1 D. of myopia still uncorrected, and added to that 
1 D., gives 8.5 D. the total myopia present. 

If the myopia be of low degree, the test without a 
lens will show either no distinguishable movement of the 
light in the pupil [for myopia of 1 D.] or movement in 
the pupil against the movement of the light on the face 
[for myopia of less than 1 D.] . In the former case, the 
test is to be repeated with very weak convex and concave 
lenses [0.25 D. or 0.50 D.]. The convex will give a 
movement of the light in the pupil with the light on the 
face, and the concave a movement against the light on the 
face. 

If the movement is found to be against the light on 
the face to start with, the convex lenses are to be tried, com- 
mencing with 1 D. lens which will cause the movement 
with the light on the face, and will show, therefore, that 
the refraction is myopia and not emmetropia, or low 



MYOPIA. 93 

hyperopia. The weaker lenses are then to be tried and the 
one which causes i D. of myopia thus ascertained. Since 
this lens is added to the myopia of the eye to cause i D. 
of myopia, it must be subtracted from i D. to find the 
amount of myopia originally in the eye ; the difference 
between it and i D. being the myopia present. 

Thus, in a case of myopia of 0.50 D., the light will be 
found to move against the light on the face, without any 
lens or with a 0.25 D. convex. But it will be found to 
move with the light on the face with a convex 1 D. or 
0.75 D., and with an 0.50 D. the movement should be in- 
distinguishable. The convex 0.50 D. then, causes 1 D. of 
myopia ; and subtracting it from 1 D., leaves 0.50 D. the 
degree of myopia existing in the eye. 

Emmetropia. — In emmetropia, on the first trial, the 
light in the pupil is found to move against the light on the 
face, and rapidly. With convex lenses it is found that 
the 0.75 D., or anything weaker still allows this movement 
against the mirror; but the 1.25 D. or anything stronger 
causes motion in the pupil with the light on the face ; and 
that the convex 1 D. causes no perceptible movement. 
Hence, the convex 1 D. lens causing 1 D. of myopia, the 
eye without a lens must be emmetropic. 

Regular Astigmatism. — The test beginning as for 
simple hyperopia or myopia, as the point of reversal for 
one of the principal meridians is brought near the ob- 
server's eye, the movement of light becomes notably more 
rapid in one meridian than in the other, indicating the 
presence of this form of ametropia. When this is recog- 
nized, the lenses used are to be such as give a movement 
of the light in the pupil with the light on the face in all 
meridians. Thus, if the eye has been hyperopic, the con- 
vex lenses used before the eye must be increased in strength 



94 APPLICATION WITH THE CONCAVE MIRROR. 

until the movement with the light on the face occurs in all 
directions. Or, if the eye is myopic, the increase of 
strength in the concave lenses must stop so soon as any 
movement is seen in the pupil against the movement of 
light on the face. And the lens which causes this must be 
replaced by a weaker one that just allows movement with 
the light on the face in all meridians. 

The lens aimed at is the one which will bring the 
point of reversal for the least myopic meridian, just to the 
surgeon's eye, i metre from the patient. If this is exactly 
attained, there will be in that meridian no perceptible 
movement of light and shadow, but the movement in the 
other principal meridian will still be with that of the light 
on the face. 

When this lens has been found, the original source of 
light, which, up to this time has been kept at as great a 
distance as possible from the mirror, is to be brought closer 
to the mirror, so that the image of it formed at the conju- 
gate focus in front of the mirror will be removed farther 
from the mirror and closer to the patient's eye. 

The lens before the patient's eye brings the point of 
reversal for the least myopic meridian to the eye of the sur- 
geon and necessarily places the point of reversal for the 
more myopic meridian somewhere between the surgeon and 
patient. The object of bringing the original source of 
light nearer to the mirror is to carry the immediate source 
of light to the point of reversal for this more myopic 
meridian. As the light approaches its proper position, the 
area of light in the pupil becomes more and more band-like 
in form, being most distinctly so when the immediate 
source of light corresponds with the point of reversal for 
the more myopic meridian. 

When this is attained, the direction of the band is to 



REGULAR ASTIGMATISM. 95 

be carefully noted as indicating the direction of the princi- 
pal meridian of least myopia. This direction having 
been determined and recorded, the original source of light 
is again moved as far away from the mirror as possible, and 
measurement of the refraction in the least myopic meridian 
completed as for a case of simple myopia. 

Then, the lenses are so changed as to bring the point 
of reversal for the more myopic meridian to the surgeon's 
eye i metre distant from the patient : and the lens that is 
found to do this, shows by the addition of I D. to a con- 
cave, or the subtraction of i D. from the strength if 
convex, the amount of myopia or hyperopia in the second 
meridian. The difference between the two meridians is 
the amount of astigmatism. 

When it has thus been ascertained, the cylindrical 
lens correcting it is to be placed before the eye and with it, 
the spherical lens, which will bring the point of reversal to 
a distance of one metre. The trial is then repeated and if 
the point of reversal be found at the surgeon's eye for all 
meridians of the pupil, the determination already made is 
accurate. If, however, there be found distinct movement 
in the visual zone in some one direction, while movement 
in the principal meridian perpendicular thereto is abolished, 
the cylinder selected does not perfectly correct the astig- 
matism. 

If this movement be in one of the principal meridians 
as previously determined, [in the direction of the axis of 
the cylinder placed before the eye, or at right angles to 
that axis] the cylinder has been properly placed, but is too 
strong or too weak, and its strength must be diminished or 
increased according to the indications of the movement. If, 
however, the movement appears to be in a meridian different 
from either of the principal meridians as at first determined, 



96 APPLICATION WITH THE CONCAVE MIRROR. 

[different from the direction of the axis of the cylindrical 
lens before the eye, or the principal meridian at right 
angles to that axis] the axis has not been properly placed 
— does not conform exactly to the direction of the princi- 
pal meridian. 

If the cylindrical lens before the eye is of the right 
strength or too weak, its axis needs to be turned slightly 
toward the axis of a similar cylinder, which will correct 
the remaining astigmatism. If the cylindrical lens already 
before the eye is too strong, its axis needs to be turned 
toward the proper position the axis for a cylindrical lens of 
the opposite kind that would correct the astigmatism. 
Such a change in the direction of the axis of the cylinder 
is to be made, and the test repeated until the correction of 
any remaining astigmatism conforms exactly with the 
direction of the lens before the eye. This remaining 
astigmatism must be corrected by a change in the strength 
of the lenses employed. 

For example : suppose an eye to have compound hy- 
peropic astigmatism corrected by +4 sph. O +2 cyl. axis 
90 . The first inspection of the pupil shows the light 
moving against the light on the face in all meridians. 
Convex lenses 2 D. and 4 D. placed before the eye show the 
same thing. Convex 6 D. shows the light moving against 
the light on the face from side to side, but with it in a ver- 
tical direction. It thus becomes evident that astigmatism 
is present. Still stronger convex lenses are to be tried. 
The 8 D. lens shows movement in the pupil with the light 
on the face in all meridians. The 7 D. lens shows move- 
ment very indefinite or indistinguishable in the horizontal 
meridian, but clearly with the light on the face in the ver- 
tical meridian. This lens then, brings the point of rever- 
sal for the less myopic [more hyperopic without the lens] 
meridian to the surgeon's eye. 



REGULAR ASTIGMATISM. 97 

The next step is to bring the original source of light 
closer to the mirror so as to cause the immediate source of 
light to fall at the point of reversal for the more myopic 
[less hyperopic] meridian, which will now be one-third of 
a metre from the patient's eye. To do this, [supposing 
that the mirror has a focal distance of one-quarter of a 
metre, ten inches] it will be necessary to bring the source 
of light to within two-fifths of a metre of the mirror. That 
is, the immediate source of light to be at one-third of a 
metre from the patient, must be at two-thirds of a metre 
from the mirror corresponding with 1.5 D. of focusing 
power. The total focusing power of the mirror being equal 
to 4 D., the light must be so placed that the divergence of 
its rays will correspond to 4-1.5=2.5 D. That is, the 
light must be two-fifths of a metre from the mirror. When 
the light is in this position, the area of light in the pupil 
will assume the most distinct band-like appearance, run- 
ning in the direction of the principal meridian of least 
myopia [greatest hyperopia] in this case horizontal. 

Having thus determined the direction of the principal 
meridians, one being known from the direction of the other, 
the original source of light is again placed back of the 
patient as far as possible, and the refraction in the horizon- 
tal meridian carefully tested by trying first the +6.5 D. 
spherical lens, and then the +7.5 D. spherical lens before 
the eye, the former of which shows the movement in a 
horizontal meridian against the light on the face, and the 
latter a movement in the same meridian with the light on 
the face, thus fixing the refraction of that meridian as 7 D. 
— 1 D.=6. D of hyperopia. 

Weaker convex lenses are then to be tried until it is 
found that with the 5.5 D., the light moves with the light 
on the face in the vertical meridian, and with the 4.5 D. it 



yb APPLICATION WITH THE CONCAVE MIRROR. 

moves against the light on the face in the vertical meridian, 
while the 5 D. gives no distinguishable movement in that 
meridian, showing that 5 D. — 1 D. = 4 D. is the amount of 
hyperopia in the less hyperopic meridian. The difference 
between the two then is found to be 2 D., the amount of 
regular astigmatism present. 

The surgeon will then place before the patient convex 
5 D. spherical with convex 2 D. cylindrical axis vertical ; 
and on again trying the test, will find that he is at the 
point of reversal for all meridians. But if on placing the 
cylindrical lens he makes a slight error in the direction of 
its axis, placing it say at 5 one side from the vertical, he 
will find on testing the eye some appearance of astigmatism 
with its axis inclined several degrees in the other direction 
from the vertical. And, to get rid of this astigmatism, 
he has to move the axis of the cylindrical lens to its proper 
position, pushing it towards the axis of a convex cylinder 
that would be required to correct this remaining astigma- 
tism. 

If the case be one of slightly myopic or high mixed 
astigmatism, the first inspection may show a movement 
with the light on the face in one direction, while the move- 
ment is against the light on the face in the other meridian. 
This, of course, will indicate at once the presence of astig- 
matism. The fact that it may occur makes it important 
that the first observation on the pupillary movements 
should include the movements in different meridians. 

With the concave mirror [the immediate source of 
light necessarily lying as far in front of the mirror as its 
principal focus or even farther] if the astigmatism be of 
quite low degree, when the least myopic point of reversal 
is at the surgeon's eye, the more myopic point of reversal 
will be at the immediate source of light, or even closer to 



REGULAR ASTIGMATISM. 99 

the mirror without any change in the position of the origi- 
nal source. Thus the most distinct band-like appearance 
of the light in the pupil, the clearest difference between 
the movement against the light on the face in one meridian 
and the indefinite movement in the other meridian will be 
attained without bringing the original source of light any 
nearer to the mirror than its usual position. This must be 
borne in mind for low degrees of astigmatism. 

Aberration and Irregular Astigmatism. — With the 
concave mirror and the need of bringing the point of 
reversal to a fixed distance from the patient's eye, the 
measurement of the amount of aberration and irregular 
astigmatism becomes very much more tedious and difficult 
than with the plane mirror, though not impossible. It is, 
however, not difficult to detect the presence of such defects, 
and to ascertain which portion of the pupil they occupy, 
and which portions being comparatively free from them are 
available as a visual zone. As to the importance of such a 
study of the pupil, what has been said in the chapter on 
the plane mirror will equally apply here. 

Measurement of Accommodation. — With the aid of 
lenses, usually concaves, the near point of accommodation 
may be brought to the required distance of one metre from 
the eye and the amount of accommodation thus measured. 
The arrangement of the patient's and surgeon's eyes, and 
of the points to be looked at is the same as that described 
in connection with the measurement of accommodation with 
the plane mirror. It is, of course, impossible to make the 
approximate determination of the accommodation with the 
concave mirror by the surgeon approaching the eye of the 
patient. He must rely on a change of lenses to bring the 
point of reversal to the fixed distance of one metre. 



CHAPTER VIII. 

GENERAL CONSIDERATIONS. 

Apparatus. — In the chapter of the Conditions of Accu- 
racy, something has already been said as to the apparatus 
by which these conditions are best complied with. The 
requirements, to meet which the apparatus for skiascopy 
is to be adapted, are that, it shall furnish the conditions 
necessary to the greatest accuracy, and that it shall facili- 
tate the finding of the lens that will bring the point of 
reversal to the surgeon's eye. 

The Mirror. — As has been stated, the essential point 
in the mirror is to have the sight hole free from reflections. 
This may be obtained by having the glass thin if the sight 
hole is cut through it, having its margin free from chip- 
ping, beveled as little as possible, and thoroughly blackened 
with a dead black. 

If the sight hole is not cut through the glass, but is 
merely an aperture in the silvering, the glass may be much 
thicker and there is no ground glass to deal with. The 
difficulty with such a mirror is in keeping the exposed 
glass at the sight hole clean. Unless great care is taken 
in preserving it from dust, and care in removing any that 
falls upon it, there will be a ring of dust in the periphery 
of the sight hole, which will reflect more light than would 
the ground glass. And, it is difficult to keep this space 
entirely clean without chipping into the backing of the mir- 
ror in such a way as to cause annoving reflections. But, 

(100) 



THE MIRROR. 101 

however difficult, it is essential to have the sight hole free 
from reflections. 

The size of the mirror will depend somewhat upon 
the purpose for which skiascopy is to be used. If the mir- 
ror is to be employed to measure refraction of all kinds ; to 
show the movement of light in the pupil with high uncor- 
rected hyperopia or myopia, it must be large ; to give the 
range of movement for the immediate source of light that 
is necessary to render evident the direction of movement 
in the pupil, when that movement is slow and the illumi- 
nation of the light area is comparatively feeble. 

The disadvantage of a large mirror is that it gives a 
large area of light on the face, especially when, as with the 
plane mirror, the original source of light is brought close 
to it. And in this large area of light on the face only the 
light reflected by a small portion of the mirror immediately 
surrounding the sight hole is of any use when the point of 
reversal is near to the surgeon's eye [see page 30 for discus- 
sion of limits of the part of the retina visible in the pupil] . 
With a small mirror, making a small area of light on the 
face, it is easier to keep this upon the eye than it is to keep 
properly directed the similarly limited portion of a largearea. 

On this account, where skiascopy is used after an ap- 
proximate estimate of the refraction has been made by the 
ophthalmoscope or other means, quite a small mirror is 
found convenient. By a large mirror is meant one from 
35 to 50 mm. in diameter. By a small mirror is meant one 
under 20 mm. in diameter. The mirror, or at least the 
opaque back that carries it cannot be well reduced to less 
than 20 or 25 mm., because, if smaller than this, it will 
admit light to the eye from the original source through the 
space around the mirror, and such light, though not so 
annoying as a reflection at the sight hole, is a serious hin- 



102 



GENERAL CONSIDERATIONS. 



derance in the application of the test. The mirror plate 
then, must be large enough to shade the eye. 

A large mirror having a metal cap with an aperture of 
from 10 to 15 mm. in diameter, that can be slipped before 
the face of the mirror or turned back at pleasure, will an- 
swer for all sorts of testing. Such a mirror 2 is shown in 
figure 23. As already indicated in Chapter III, the sight 
hole should be from 2 to 3 mm. in diameter. 




The handle of the mirror should be rather broad, so 
that a very slow, even rotation can be secured ; for as the 
point of reversal is approached, the magnified movement in 
the pupil becomes so rapid that only by moving the mirror 
slower, and making excursions of very slight extent can 
this apparent motion in the pupil be readily followed. 
This difficulty of causing the immediate source of light to 
move slowly enough is diminished in proportion as the 
immediate source of light is brought closer to the mirror. 

The Shade. — The shade that covers the original source 
of light should extend far enough above and below to pre- 
vent the escape of any considerable amount of light into 
the room. Where an argand burner is used as the source, 

2 Made at my suggestion by Wall and Ochs of Philadelphia. Another form 
is described by Dr. James Thorington, -Philadelphia Polyclinic, 1893, page 
329. 



THE SHADE. 103 

a cylindrical shade is needed, 20 to 25 cm. long, and having 
a diameter of 6 or 6.5 cm., slightly greater than that of the 
chimney used so as to allow a free current of air between 
the shade and chimney, and thus diminish the heat from the 
flame. An asbestos shade as proposed by Dr. J. Thoring- 
ton (Ann. of Ophthalmology and Otology, 1895, p. 5) is to be 
preferred to metal on account of intercepting the heat of 
the name. 

The aperture of about 5 mm., or larger, if the test is 
not intended to be very accurate, should be opposite the 
brightest part of the flame, which ought to be broad enough 
to allow of slight change of position of the surgeon with 
reference to it without its becoming hidden by the shade. 

The Lenses. — Ordinarily these are taken from the 
trial case and placed in a trial frame before the eye. It is 
important to have them clean and comparatively undam- 
aged by scratching. The trial frames should be such as to 
support the lenses well up before the eye and with their 
centres before the centres of the pupils. They must also 
be far enough away from the face to escape the touching 
of the lashes, and to prevent the condensation of moisture 
upon them. The interruption of the red reflex from the 
pupil by such an occurrence prevents the satisfactory ap- 
plication of the test, and may be quite puzzling, because 
the reason for the obscuration is not immediately apparent, 
and it may be ascribed to opacities within the eye. 

Support of Lenses. — The trial frames have the advan- 
tage over other supports for lenses to be presently men- 
tioned, that they keep a constant position with reference to 
the patient, so that a slight movement of the patient's 
head does not carry his eye away from the centre of the 
lens to its periphery or beyond. 

When the surgeon has learned to estimate by the 



104 



GENERAL CONSIDERATIONS. 



rapidity of movement of the light in the pupil the amount of 
ametropia remaining uncorrected, by following the plan 
here laid down of considerable intervals between the lenses 
until an approximation of the required lens has been made, 
the number of changes of lenses for any case is not neces- 
sarily great. So that for any one who does not employ 
skiascopy on large numbers of patients daily, the trial 
frame and lenses will be found satisfactory. 

Special series of lenses mounted in revolving disks 
have been arranged by Haines {Ophthalmic Review, 1886, p. 
282), Doyne, Couper, Burnett (Trans. Am. Ophthalmol. Soc, 
y^fimSi^ 1888, p. 223), Wurdemann and others, to save 
time by facilitating the changes to the lens 
required. Some of these have been designed 
for the patient to make the change of lens 
under direction of the surgeon, and others to 
give the surgeon himself constant control of 
their movements. 

One of the simplest arrangements is the 
u hand-skiascope " of Wurdemann (American 
Journal of Ophthalmology, 1891, page 223) 
shown in figure 24. The lenses are inserted 
in a sheet of hard rubber which the patient 
holds by the handle, bringing before his eye 
the lens the surgeon may indicate. 

In an instrument suggested by the writer 
the disk is rotated by a rod one metre long and 
attached by a universal joint so that it drops 
out of the way when not in use. 

The lens series runs from 7 concave to 7 
convex spherical with 0.5 D. intervals, requir- 
Fig. 24. ^ n g. tQ k e SU ppi eme nted by lenses in the trial 
frame for high hyperopia and myopia, or astigmatism. 



\ 



SUPPORT OF LENSES. 105 

An ingenious piece of apparatus having a complete 
series of lenses, both spherical and cylindrical, arranged for 
the purpose has been devised by Lambert (Trans. Amer. 
Ophthalmol. Soc, 1894, p. 196). It has the lenses arranged 
in two disks for the special lenses, and detachable slides 
for the cylinders, enabling the surgeon to reach the 
lenses wanted quickly. To be compelled to run over the 
whole lens series to find the one sought, would be a way 
of consuming time rather than saving it. A series sufficient 
for the approximate testing of the majority of eyes may 
save time where many are to be tested, especially if the 
concave mirror be employed. 

Meridian Indicators. — In working with lenses in the 
graduated trial frame one may refer to its graduation to as- 
certain the direction of the bands of astigmatism. But 
in the darkened room this is not convenient. To meet 
this want, Thorington (Medical News, March 3rd, 1894) 
and Prince (Ophthalmic Review, July, 1894) have suggested 
disks specially graduated for the purpose. The former, 
figure 25, called an axono meter ; the latter, figure 26, called 
an inclinometer. 





Fig. 25. Fig. 26. 

A Distance Measure. — Where the concave mirror is 
employed, the distance remaining fixed throughout the test, 

8 



106 GENERAL CONSIDERATIONS. 

it is only necessary that the surgeon should properly place 
himself at the beginning, and retain his position. He can 
then dismiss the consideration of the distance, and consider 
simply the changes made in the lens before the eye. 

With the plane mirror, no measure is necessary where 
the test is used only to approximate the refraction, the sur- 
geon soon learning to guess at the distance close enough to 
be within 0.25 D. of the amount of myopia present with 
the lens fixed upon. But, for exact measurement, it is con- 
venient to have something to measure from the patient's 
eye to the surgeon's. This may be either a tape attached 
to the trial frame or lens disk (Burnett), picked up and 
held to the surgeon's eye when the test is completed, or the 
ordinary metre stick. In either case, it is convenient to 
have the measure graduated in dioptric focal lengths de- 
scribed by the writer in the Medical News, June 27, 1885. 
The graduation should begin from the end that is applied 
to the patient's eye. 

Mydriatics. — In making any measurement that is to 
have positive significance, the first essential is that the 
quantity to be measured should be fixed. When the refrac- 
tion of the eye varies from moment to moment, it is impos- 
sible to make a valuable measurement of it by any method. 
When it is liable to vary from moment to moment, there is 
a liability to error, due to such variations. Believing that 
when consulted as to an error of refraction or its effects, the 
ophthalmic surgeon should ascertain its degree with exact- 
ness and certainty, the writer is accustomed to employ a 
mydriatic so as to secure complete paralysis of accommoda- 
tion in the great majority of cases under 50 years of age. 

While any of the true mydriatics, atropin, daturin, 
duboisin, hyoscyamin, or scopolamin, will give a satisfac- 
tory paralysis of accommodation, if a mydriatic is used solely 



MYDRIATICS. 107 

for diagnostic purposes, homatropin should be selected on 
account of its briefer period of recovery. Properly used, it 
is for practical purposes of diagnosis, as reliable as any 
mydriatic we possess. To secure paralysis of accommoda- 
tion, therefore, four to six drops of a two to four per cent, 
solution of homatropine hydrobromate are to be instilled, 
one drop at a time at intervals of five minutes, about an 
hour before the test is to be applied. 

To apply skiascopy with the greatest ease, requires a 
pupil moderately dilated. Like other methods for the 
measurement of refraction, it will not give as accurate re- 
sults if the pupil be narrow ; and, on account of the aber- 
ration and irregular astigmatism that usually exist near the 
margin of the lens and cornea, the wide dilatation of the 
pupil introduces factors of confusion. The need, then, for 
a dilated pupil is about the same with skiascopy as for the 
use of the refraction ophthalmoscope or the test lenses, ex- 
cept that skiascopy is slightly more at a disadvantage when 
the pupil in the dark room is less than four millimetres in 
diameter. When this is the case, sufficient dilatation can 
be obtained, with the least inconvenience to the patient, by 
placing in the eye a drop of a two or four per cent, solution 
of cocaine, thirty to fifty minutes before using the test. 

Relative Advantages of Plane and Concave Mirrors. 
— The difference in methods of using most efficiently the 
plane and concave mirrors have caused most surgeons to 
habitually employ the one or the other, and to depend upon 
it almost entirely for practical purposes. Either, properly 
used, will meet the requirements of practice. 

In Astigmatism, the plane mirror is capable of de- 
termining with greatest accuracy the meridian of greatest 
myopia, but not the meridian of least myopia. On the 
other hand, the concave mirror fixes with greatest accuracy 



108 GENERAL CONSIDERATIONS. 

the meridian of least myopia, but not that of greatest myopia. 
But, for regular astigmatism — the astigmatism that can 
be fully corrected by a cylindrical lens, or by any com- 
bination of cylindrical lenses — the principal meridians are 
always perpendicular to each other, so that for practical 
purposes, it is only necessary to accurately locate one of 
them ; and it is a matter of indifference which one this 
shall be. 

In positive aberration, the focusing of the light upon the 
retina being such that the light area has the sharpest out- 
line when the immediate source of light is closer to the eye 
than the point of reversal, this can only be effected by the 
concave mirror, which, therefore, has so much advantage 
over the plane mirror. It is of some practical importance 
in a few cases, in which the aberration invades the visual 
zone. 

With negative aberration, the advantage lies with the 
plane mirror, but is of still less practical importance on ac- 
count of the smaller number of cases of aberration of this 
kind. 

With a concave mirror, the distance from which it can 
be used with advantage is fixed, and the surgeon being able 
to readily check his position, by a mark on the neighboring 
wall or some similar device, there is no need of the meas- 
urement of the distance between the surgeon and the 
patient ; but all changes of the movement of the light in 
the pupil must be effected by a change of the lens before 
the eye. On the other hand, the plane mirror can be used 
from any fixed distance, but allows a variation of the dis- 
tance of the surgeon from the patient, and therefore requires 
the fewer changes of the lens before the eye. 

This latter advantage of the plane mirror over the con- 
cave mirror may not seem very great, but when it comes to 



ADVANTAGES OF PLANE AND CONCAVE MIRRORS. 109 

the accurate determination of the refraction, requiring re- 
peated inspections of the light movement in the pupil from 
within and from beyond the point of reversal, it is really 
quite important. The disadvantage of the concave mirror 
may be lessened by the employment of some such arrange- 
ment of lenses in a disk as has been referred to in a preced- 
ing section. But, even in this case, the fact that there is a 
complete break between the appearances presented by one 
lens, and the appearances present by the use of the lens next 
stronger or weaker, makes the information obtained less 
valuable and satisfactory than that derived by the move- 
ment of the surgeon's eye from one position to another, 
which allows him to watch the different appearances of 
light and shade as they pass gradually into each other. 

Perhaps the greatest advantage of one over the other 
is that of the plane mirror over the concave in thus making 
possible the more complete study of aberration and irregu- 
lar astigmatism. 



INDEX. 



ABERRATION, 17, 41, 56, 59, 84, UACIAL light area 23, [25 

99, 108. * Fantoscopie, 11 

Accommodation, 86, 99, 106 Focal lengths, 106 

Accuracy 36, 40, 45,49 Forbes, 9 

Advantages, 5, 107 Form of light area, 32, 47, 53 

Apparatus, 15, 37, 71, 100 Fundus-reflex test, n 

Apparent movement, ...20,22. 26, 31, 
50, 55, 

Applications, 71, 89 

Area of light. ..23, 25, 26, 32, 47, 53 T T a TN ro iozL 

Artificial eye, 18 H S ?' ; ^ ?? 

Astigmatism,... 7, 17, 3», 41, 46, 56, Hirti^!!!^V""ZZrZZ" 

Ax 7 o 6 nometer; 105 Hypermetropia 7"'^ 26, 72, 9 o 

BT ILLUSTRATIONS,. .20, 23, 25, 31, 

AND appearance,... 7, 47, 78, 94 1 32, 48, 49. 55, 57, 61, 62, 67, 69, 

Bowman, 7, 10, 47 85, 102, 104, 105. 

Brilliancy of light, 33, 36 i mme diate source, 23, 38 

Burnett, 104 Inclinometer 105 

C^ . „~„ « Indicators, 105 

H o?u LEY ' 9 Inverted, , 20, 61 

Chibret, 9' 11 Irregular astigmatism, 7, 41*56, 84. 

Concave mirror, 9, 24, 39, 40, 44, 5 1 . 99 

54, 62, 64, 89, 109. 

Conditions of accuracy, 36 JACKSON, 9, 10, 81, 106 

Conical cornea, 7,65 J Juler, 9 

Contents, 3 

Couper 7 , 104 1ZERATOSCOPIE,., 10 

Cuignet 8,9, 10 ^ Koroscopie, 11 

DARK ROOM, 36 J AMBERT, 105 

Derby, 37 L Landolt, 11 

Difficulties 11 Learning the test, 13 

Dioptric Scale 106 Lenses, 103, 104 

Dioptroscopie, 11 Light area,. .22, 25, 26, 28, 32,38, 53 

Direction, 47, 55, 78, 97 Light source, 23, 36, 71, 89, 102 

Distance, 42, 43, 49, 89, 105, 108 

Donders, 7, 8 JVAAGNIFICATION of retina,. ..30 

Doyne, 104 iVl Mengin, 8 

Meridians, 46, 81, 97, 105 

EGGER 11 Mirror, 37, 100 

Emmetropia, 24, 26, 76, 93 Morton, 9 

Enlargement, 30 Movements of light, 23, 25, 26,28, 

Erect, 20, 61 31, 53, 61. 

(in) 



112 INDEX. 

Mydriatics, 106 Retinal area, 23, 25, 28, 32, 38 

Myopia, 7, 19, 24, 26, 74, 91 Retinal enlargement, 30 

Retinophotoscopie,... 11 

jNJAME 10 Retinoscopy, 10 

1> Negative aberration,... 42, 59, Retinoskiascopie, 11 

6 3, Io8 « Reversal, 19, 34, 46 

pvBLIQUITY of lens, 70 QCISSORS movement, 69 

W Oliver, 11 O Shade, 37, 102 

Optical principles,... 19 Shadows, 26, 32, 47, 56, 60, 69 

Original source of light 23, 38 Shadow-test, 11 

PARENT, 9, 10, 11 Sight-hole, 37, 100 

r Plane mirror,.. 9 , 23, 38,40,44, |£Sf ™™ r >" 

5o, 53, 60, 63, 71, 109. • sS aS coov 

Point of Reversal, 21, 34, 46 ^ lasco VJ r 

Position, ..40, 43, 49, fi\ §9 1°^ ° f llght ''" * 3 ' 36 ' 38 ' 7I ' 



Smith, Priestley, 11 

11 



Positive aberration,. .4 1, 59,60, 108 
Practial application, 1^, 71, 



Story, 9 

Study of test, 13 



Preface ^^^"' * "» ^ Symmetrical aberration, 41, 42, 58, 

™ ace> ■••; 59,60,63, 108. 

Prince, 105 ^' ' J ' 

Principal meridians, 46. 81, 97 T^HORINGTON, 102, 103, 105 

Pupillary shadows,... 26, 32, 47, 56, 1 

60, 69. 
Pupilloscopie 11 T 1MBRASCOPY, 11 

RANDALL, 43 
Rate of movement,. ..28, 31, 66 VISUAL ZONE, 3, 59 

Real movement, 22, 28 

Regular astigmatism,... 7, 17, 38, 41, \*7EILAND, 81 

46, 76, 93, 107. VV Wiedemann, 104 



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